A primer is a preparatory coating put on materials before paint

Paint

Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...

ing. Priming ensures better adhesion

Adhesion

Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....

 of paint to the surface, increases paint durability, and provides additional protection for the material being painted.
Primer is a paint product that allows finishing paint to adhere much better than if it were used alone. For this purpose, primer is designed to adhere to surfaces and to form a binding layer that is better prepared to receive the paint.