To borrow an example from Fr. Spitzer, an infinite regression of causation (arguing that everything, without exception, has been caused by something else, .i.e. has a creator, including God) is like: You need a book (existence), and go to get it from person Z. Person Z tells you, “I don’t have it, ask Y” and on and on it goes, where EVERY person you asked to hand the book over to the next (so it could pass down to you!) simply kept pointing to another person before them, shrugging ("Ask A, B, C, D). Now imagine there was an INFINITE LINE of people, each of whom needs to get the book from someone ELSE before they can hand it to the next person: Would you ever get that book? Nope.
But here you are, holding the book (your own existence) and trying to find WHO passed it down to you. That means there isn’t an infinite line of book-givers/lenders behind you. As we’ve seen, if there were, the result would be NO book. Not unless SOMEONE just had the book and didn’t need to point to another could this process (of book-giving that has led to you holding a book) ever start.
So even without knowing anything else about this first book-giver except that you have received a book, you can know two things: (1) A book-giver exists, given you’re holding a book; (2) This first-book giver did not get the book from ANYONE else (or he could never hand it down to you). So you know the first book-lender OWNS the book in an absolute sense: He doesn’t get it from a prior cause, he just has the book by his very self/nature, which is how he’s able to start the chain of book-giving/lending that has led to the book you’re holding.
Therefore, we can know from our basic existence that a truly uncaused cause exists and it is the reason, in the ultimate scheme, of all the “somethings” we know. From this, other chains of reasoning lead to a few other conclusions about what else this ultimate book-giver must be for any of this to be how it is.