See Aristole's Categories: The Four-Fold Division.
[Cat,1a16] Of things that are said, some involve combination while others are said without combination. Examples of those involving combination are: man runs,
man wins; and of those without combination: man, ox, runs, wins.
Here A introduces the basic concept of predicate ("thing said"): predicates are expressed by words that can only be used in connections with others (subjects), like in "man runs".
But a predicate can be predicated of other predicates that are "subordinated" to the first one:
[Cat,1b10] Whenever one thing is predicated of another as of a subject, all things said of what is predicated will be said of the subject also. For example, man is predicated of the individual man, and animal of man; so animal will be predicated of the individual man also — for the individual man is both a man and an animal.
In a nutshell (but the interpretative issues are many) we have a four-fold distinction between:
universal substances, particular substances, universal accidents and particular accidents.
By focusing on Aristotle's illustrations, most scholars conclude that beings that are said-of others are universals, while those that are not said-of others are particulars. Beings that are present-in others are accidental, while those that are not present-in others are non-accidental. Now, non-accidental beings that are universals are most naturally described as essential, while non-accidental beings that are particulars are best described simply as non-accidental. If we put these possibilities together, we arrive at the following four-fold system of classification: (1) accidental universals; (2) essential universals; (3) accidental particulars; (4) non-accidental particulars, or what Aristotle calls primary substances.
A universal substance is "man" (referring to humans, the universal), while a particular substance is "this man" (e.g. Socrates, the individual).
The primary substances are particulars: this man, this horse, while universals (humanity, horseness) are substances in a secondary sense.
A particular (the individual man: Socrates) can be a subject in the predication relation but cannot be used as a predicate: it is "not said of a subject" [1b9: "Things that are individual and numerically one are, without exception, not said of any subject"].
A universal (animality) can be predicated of subjects, both of particulars (Socrates is a (rational) animal) and of universals of "lesser ontological level" (humans are animals): it is "said of a subject".
Substances are not accidents (properties) and thus they are not "present in" or "being in".
On the contrary, according to A, attributes are "present in". As for the case of substances, we have accidents that are universal and accidents that are particular.
An universal accident, like "whiteness" can be predicated of some universal substance (e.g. man): thus, it is "said of a subject".
What is an "accidental particular" ?
It is not easy to find a good example: we can try with "this whiteness", meaning the specific shade of white belonging to Socrates' hairs.
In this case, we have an accidental particular (a particular instance of a quality) belonging of a non-accidental particular (an individual); this case is that of "being in a subject" (the particular quality is not a substance) and "not said of a subject", because it is not possible to use particulars (the individual instance of the quality "whiteness") as predicates.
Perhaps a more intuitive example is the particular whiteness that some object has. If there are non-substantial particulars, then Socrates' whiteness is a numerically distinct particular from Plato's whiteness. Hence, Socrates' whiteness cannot exist without Socrates.