Well, beyond what is in Aquinas, you can search "intentionality" on Feser's blog and find a lot of material; you could also consult his book on philosophy of mind.

The phenomenon of intentionality is supposed to link various connected ideas. There is the idea that intentionality is 'aboutness'; a thought which is intentional, in this sense, is about something in the world. My thought that the Eiffel Tower is in France is about the Eiffel Tower. Relatedly, the thought has what we might call representational content. It represents the world as being a certain way and is answerable to the world's being that way. It is true or false according as whether or not the world is that way. That is a sense in which it is about the world.

There are also grammatical markers of descriptions of intentional states, mental states with this sort of content. In particular, intentional states are attributed to people by way of expressions which form intensional (with an s) contexts. An extensional context is a context into which I can swap coextensive terms without altering the truth of the sentence. So take the sentence "The Eiffel Tower is in Paris", and consider the corresponding context "______ is in Paris". This is an extensional context; if x = y (that is, if x and y are the same object, if "x" and "y" are names for the same object), then x is in Paris if and only if y is in Paris. The Eiffel Tower is the tallest building in France. So the Eiffel Tower is in Paris if and only if the tallest building in France is in Paris. Not so for intensional contexts. "I believe that the Eiffel Tower is in Paris" might be true even if "I believe that the tallest building in France is in Paris" is false. "I believe that ________" is an intensional context, into which we cannot substitute coextensive terms salva veritate.

Different philosophers see different features of intentionality as being fundamental or interesting or challenging.

There's a lot more to be said on the topic, and it's a topic of controversy in philosophy. Some philosophers hope to naturalize or eliminate intentionality. Others think it is irreducible or ineliminable. The reason it is thought difficult to reduce is that it is difficult to see how any helpful candidate for the reduction could have the representational content which an intentional state has.

What don't you understand? And why do you think you need to talk about intentionality in RCIA?

John Searle's "What Is an Intentional State?"  would give you some sense of the terrain and is not, I think, too difficult. I have a pdf of this; if you would like it, send me a PM.

Another accessible intro text which touches on the topic, albeit briefly, is Madden's Mind, Matter, and Nature. If you are trying to present arguments against materialisms to teenagers, you may appreciate the other material in there.