A primer gets you ready for what comes next. You could use one kind of primer when you are learning to read, or another kind when you are preparing to paint a room.
Any way you use the word primer, it's a first step or preparation for something else. There's primer you use before painting, and an engine primer that prepares a machine to be started. In these examples, primer rhymes with timer. When primer describes an introductory textbook, it rhymes with dimmer. In the 14th century, a primer was a prayer book. This word comes from primus, the Latin word for first.