The intake is a system of components that manages air going into the engine. It usually begins with the air filter, passes through tubing which houses air intake sensors (such as the mass air flow sensor), to the throttle body, then into the intake manifold, finally reaching the intake valve where, when the valve opens, the air will enter the combustion chamber. Unless the engine is a direct injection model, somewhere along the intake (usually after the throttle body) the fuel will be added to the mix.
The more restrictions there are in the intake system, the more difficult it is for the engine to draw in air, which reduces overall power. Sometimes a port and polish is performed to improve air flow.