Primary is a term that indicates something that is basic or fundamental (such as primary school). It can also be used to denote something of immense importance, such as a speech stating "It is of primary importance that this comes to pass." In behaviorism, a primary reinforcer is one that exists innately within an organism and doesn't have to be learned. For example, food is a primary reinforcer - you don't have to train an animal (or human!) to want food, they naturally want it.