Okay, let me try to explain it in a simple way. Do you know that feeling when you forget something important? Like the name of your friend or something you needed to bring to school? Well, a mnemonist is someone who is really good at remembering things by using special tricks called mnemonics.
Mnemonics are like memory aids that help you remember something better. For example, you can use the acronym HOMES to remember the names of the Great Lakes: Huron, Ontario, Michigan, Erie, and Superior. Or you can visualize a funny picture in your head to help you remember a name, like imagining a cat sitting on top of your friend's head if their name is Catherine.
So, a mnemonist is someone who uses these tricks to remember things better. They have trained their brain to be really good at it. It's like a superpower!