Mathematical alphanumeric symbols are like toys for mathematicians. Just like how you have toy blocks and how they look different, these symbols look different too. They have a special job - to help mathematicians write complicated equations and formulas in a way that is easy to understand.
There are different types of mathematical alphanumeric symbols. Some look like the letters you learn in school, like A, B, C. Others look different but still have a special meaning, like ∑ and ∫. It's like how the block with the letter A on it means something different than the block with the letter B on it.
All these symbols have rules for how they can be used, just like how you have rules for how you can play with your toys. They help mathematicians talk to each other and understand really complicated ideas.
So, just like how you might have a special toy that only your friends who know the rules can play with, these symbols are special toys for mathematicians. And every symbol has a special job to help the mathematicians build something amazing together.