QUICK ANSWER
An f stop is a measure of a geometric incremented pattern based upon the powers of the square root of 2 (1.4).
This gives a geometric scale of:
f1.0 f1.4 f2.0 f2.8 f4 f5.6 f8 f11 f16 f22 f32 f45
Every stop lets in half as much light as the number goes up. E.g. f1.4 will let in half as much light as f1. A camera lens that opens to f4 will need four times the amount of light as a camera lens that opens to f2 and
