Empiricism in its most general form is the claim that all we can possibly know about the world and the laws which govern it are those things which we can directly apprehend by our senses. Hence, the very idea that objects continue to exist, unchanging, when no one's observing them, or that it is a law of nature that any event must precede and cause another, cannot be observed and can neither be known.
This is a very rough approximation, because you asked for it.
Kant's argument against this, can be roughly (again, very roughly) approximated as follows: any presumption that our senses can provide us with any information about the world, already must assume that the sense itself is caused, or it would be unable to provide us with real knowledge. Thus there is at least one law of nature which cannot be observed, defeating the general claim of empiricism.
I'm not confident that this kind of stripped-down explanation can really deliver a comprehensiveaccurate account of what Kant attempted in this argument, but as long as you're aware of the limitations and dangers in trying to eliminate the details here, it should suffice. For reference, see http://www.iep.utm.edu/kantmeta/#SH1athe IEP entry on the topic, which gives slightly more detail over the version I've tried to provide here.