Ad infinitum means something that goes on and on without ever stopping. It's like when you're playing your favorite game and you don't want to stop - you just want to keep playing and playing and playing.
Sometimes, people use ad infinitum when they want to describe something that seems like it will never end. For example, if you're counting a lot of things, like jellybeans or toy cars, and you keep on counting without stopping, you could say that you're counting ad infinitum.
Ad infinitum is a big phrase that sounds really fancy, but it's really easy to understand - it just means something that goes on and on and on, like you never want it to stop!