What is valve springs?

There are valves in the head of the motor. They open to let in air and exhaust out. There is a spring around each one that keeps them closed until the camshaft forces the valve open.

Edit: Another way to put it is that the valve spring's job is to close the valve once it's opened by the valve train. The stronger the spring, the quicker it closes the valve and the higher the RPM the engine can operate without 'floating' the valves. The tradeoff is that the stronger the spring, the harder it slams the valve against the seat, reducing each's useful lifetime.