Mentat is a fictional character in the popular science fiction book series called Dune written by Frank Herbert. He is a super intelligent human being who is trained to have an extraordinary memory and the ability to do complex math in his head without the use of a calculator.
Imagine a person who is really, really good at remembering things. They can remember things they've seen or heard from a long time ago and never forget it. They can also do really hard math problems in their head like adding big numbers or multiplying really fast, even when no one else can do it.
Mentats are like the super computer in a human form. They use their intelligence to help people make important decisions and solve problems that are difficult for others. They are kind of like superheroes because they have special abilities that no one else has.
So in short, Mentats are super smart people who have amazing abilities that help them solve very complex problems. They are fictional characters popularized in the book series called Dune.