Imagine you want to learn something new, like how to play a game. But before you can start playing, you need to know the basic rules and how to set up the game. A primer is like a special book that teaches you all those basics so you can get started on the fun parts.
It's like a guidebook that explains everything step by step in simple words so even someone who's never played the game before can understand. The primer will also have pictures to show you what things look like and sometimes even some games or practice exercises to teach you the rules.
In the same way, a textbook primer teaches you all the basic concepts of a subject, like math or science or history. It's a book that explains everything you need to know to get started learning about that subject. It's usually written with simple words and lots of pictures, and it will have practice exercises or questions to help you understand the concepts better.
So just like a primer helps you start playing a new game, a textbook primer helps you start learning about a new subject.