So, you know how when you're learning math, you start with some basic rules like "1+1=2" and "2+2=4"? Those are kind of like building blocks for all the other math you'll do.
An axiom system is like a big set of building blocks for math. It's a bunch of rules that we agree to accept as true, so we can build more complicated math on top of them.
For example, one of the axioms in geometry is that two points determine a unique line. That means that if you pick any two points, there's only one possible line that goes through them. Another axiom is that all right angles are congruent - which just means they're all the same size.
These axioms might seem really obvious, but they're important because we need to agree on them before we can start doing more complicated geometry.
So, just like you start with basic math rules before doing more complicated math problems, mathematicians start with an axiom system before proving more complicated theorems. It's like a foundation for all the math that comes after it!