The camber line is an imaginary line that runs through the centre of the wing, from leading edge to trailing edge. It remains equal distance from the top of the wing to the bottom of the wing.
Camber is the measure of the curvature of a wing. In simple terms, it is a distance measurement from the chord line to the camber line. Camber is measured as a percentage of the chord. If you had a chord line that was 200cm long and the greatest measured distance from the chord line to the camber line was 30cm you would have a 15% camber.
A greater camber would equate to more lift at lower speeds, but consequently more drag at higher speeds. Think about how the application of flaps changes the flight characteristics of your airplane. With flaps you create more lift at lower speeds (your stall speed decreases), however you increase the amount of induced drag.