Mathematical platonism is like believing in cartoon characters. Just like how you believe that your favorite cartoon character like Mickey Mouse or SpongeBob SquarePants exists even though they are not real, mathematical platonists believe that mathematical objects like numbers, shapes, and equations exist in a non-physical world or realm.
While you can see a cartoon character on the TV or in a book, you cannot see mathematical objects with your eyes. But, just like how you can imagine and draw your favorite cartoon character, mathematicians can imagine and reason about mathematical objects using their minds.
There are different kinds of mathematical platonism, but they all share the belief that mathematical objects are real and independent of human thought. This means that even if humans did not exist, the mathematical objects would still exist in the same way as they do now.
Some people might believe that math is just something we invented, but mathematical platonists think that math is more like a discovery of something that already exists. It's like finding a hidden treasure map rather than drawing one up.
So, just like how you believe in the existence of your favorite cartoon character, mathematical platonists believe in the existence of mathematical objects as real and independent things.