So, it's not entirely clear what Russell is up to in the quoted sentence, but here's a thought about what's going on.
I'll start with some background (if this is all familiar to you, even better). Russell is working with a philosophical picture of the mind according to which what is directly apprehended by the mind are 'experiences' or 'sense-data'. Knowledge of ordinary objects - tables, chairs, other people - is, on this picture, inferred from encounters with sense-data. Sense-data are known non-inferentially. So, for instance, my knowledge that there's a chair in front of me is inferred from the fact that I have brownish, chair-shaped experiences in my visual field, that the object resists my touch, and so on. The fact that I have brownish, chair-shaped experiences is known to me non-inferentially - it is just 'given' to me or is directly apprehended.
Now, given this picture of the mind it's very easy to generate skeptical worries. If all you're directly acquainted with are sense-data, then couldn't your sense-data stay the same while the objects on the other end of your sense-data differ? This is just Cartesian skepticism. If you're deceived by an evil demon, or if you're a brain in a vat, or whatever, then even if your sense-data represent there being a chair in front of you there really isn't - in reality you're dreaming, or you're in a scientist's lab, or whatever. Put another way, the worry is that your inferences from your experiences to objects in the world might fail.
But Russell tells us that he doesn't buy the skeptic's argument. He tells us that there are valid inferences
from events of which I am aware without inference to events of which I have no such
awareness (HK, p. 2)
What does this mean? Well, Russell is telling us is that he thinks you can infer from your sense-data (these are the 'events of which I am aware without inference') to objects in the external world (these are the 'events of which I have no such [inferential] awareness'). That is, he thinks that the inference from 'brownish, chair-shaped visual experience' to 'there's a chair in front of me' is valid.
But the plausibility of this response hinges entirely on what Russell means by 'valid'. And I think understanding this is key to understanding the sentence that confuses you. One option is that by 'valid' he means 'logically valid' - that is, necessarily truth-preserving. An inference from P to Q is logically valid if Q is true whenever P is. So, on this reading, Russell thinks that the inference from 'brownish, chair-shaped visual experience' to 'there's a chair' is logically valid; as a matter of deductive logic alone, you couldn't have the experience without there being a chair too.
But this can't be what Russell means. First, he tells us as much:
if, then, I am ever to be able to infer events, I must accept
principles of inference which lie outside deductive logic. (HK, p. 2)
That is, if we want to infer external events/objects from sense-data events, we need more than just pure logic. Second, he tells us that
as deductive logic can show, any collection of
events might be the whole universe (HK, p. 2)
What he's saying here is that deductive logic alone doesn't let you discriminate between sense-data events and events in the external world. That is, you can't take your brownish, chair-shaped visual experience, somehow apply deductive logic to it, and get the conclusion that there's a chair in front of you (if there is) or that there's no chair in front of you (if there isn't and you're just deceived).
OK. So, deductive logic alone doesn't help us infer from sense-data to objects in the external world. Russell needs something else. He needs inferences that aren't logically valid, but which are still reliable enough to underwrite the inference from 'brownish, chair-shaped experience' to 'there's a chair in front of me'. So instead he's going to use inferences backed up by laws of nature:
All inference from events to events demands
some kind of interconnection between different occurrences. Such interconnection is traditionally
asserted in the principle of causality or natural law (HK, p. 2)
So, on this view, the inferences from 'brownish, chair-shaped experience' to 'there's a chair in front of me' is valid if there's some natural law or principle of causality that says 'if you have brownish, chair-shaped experiences (perhaps in normal circumstances), then you may infer that there's a chair in front of you'. This inference might fail (it's not logically valid, remember). But it's reliable enough to get us knowledge of the external world, which is what Russell is after.
Put another way, your inference from experience to external world is justified by there being a law of nature that says 'if you have brownish, chair-shaped experiences (perhaps in normal circumstances), then you may infer that there's a chair in front of you'. But this justification doesn't come from logic - hence your justification isn't a matter of the logically necessary features of the world. Rather, the justification comes from the non-logically necessary (i.e. contingent) features of the world, namely, the laws of nature that happen to hold in the world (remember: the laws of nature might have been different than they in fact are, so they aren't logically necessary).
Let's put all this together. When Russell says
Inference from a group of events to other events can only be justified if the world has certain characteristics which are not logically necessary
He means that our inferences from sense-data to events/objects in the external world can't be underwritten by logically necessary features of the world (namely, the laws of logic). Rather, they can only be underwritten by contingent features of the world, such as laws of nature. And obviously that can happen only if the world indeed has certain characteristics which are not logically necessary. 'Certain characteristics which are not logically necessary', in this context, means something like 'laws of nature'.
I hope that helps! If anything I've written is unclear please let me know.