What is primer and why is it used before painting?

Primer is a preparatory coating applied before the final paint on surfaces such as walls, wood, or metal. It acts as a base layer that improves paint adhesion and ensures better bonding with the surface. Using primer enhances paint durability, provides uniform coverage, reduces paint absorption, and helps achieve a smoother and more long-lasting finish. It also improves color consistency and overall appearance of the painted surface.