A primer system allows the pilot to squirt raw fuel directly into the intake manifold or into the barrel of the carburetor to aid in starting a cold engine. The primer serves the same purpose as the choke or enrichener, namely that of producing a very rich mixture during startup. Normally, either the primer or the choke will be used. Both are seldom required.
In the cockpit, the "primer" is a small, syringe-like pump that is mounted on the instrument panel, or maybe elsewhere, within reach of the pilot. Pulling the primer knob outward draws fuel into the barrel of the primer. Pushing it in squirts this fuel into the primer port on the carburetor, or into a special port on the intake manifold.