First the joke answer. In mathematics, once you understand the proof of something, we say that it is trivial. Category theory is about making things trivially trivial.

Now the serious answer. Category theory is about abstracting away the details of mathematical domains so that fundamentally parallel proofs and constructions can be seen to be essentially the same. It also allows you to create constructions that allow problems in one field to be transformed into problems in an apparently unrelated one where you might find them easier to prove.

The result is that in a new field, you recognize the category and suddenly have a whole bunch of results, and some important constructions to look at. Such as the tensor product.

And in existing fields you can do things like use homotopy theory to turn topology problems into algebra problems, where they are often easier.