Nonstandard analysis is a way to do math that lets us work with things that are really, really small or really, really big. We call these things "infinitesimals" and "infinities."
Imagine you have a bunch of candy bars, and you want to figure out how much each person will get if you divide them up equally. But what if you have a really tiny piece of candy left over that's too small to be divided? Or what if you have so many candy bars that you can't count them all? Nonstandard analysis lets us answer these kinds of questions.
It's like having a special magic tool that lets us zoom in or out as much as we need to, so we can see things that are too small or too big for regular math. We can use nonstandard analysis to study things like calculus, geometry, and probability, and it helps us understand things that were too hard to understand before. It's like having a superpower!