Values for the aperture are written as a fraction, like this:
f/1 (Large aperture; lets in a lot of light)
f/16 (Small aperture; lets in only a little light)
The aperture can be any value. But in practice, these values are the most common ones that are quoted. Moreover, these are the values used by lens manufacturers to represent the “stops”.
Notice that the number is in the denominator of a fraction. Thus, f/2 is bigger than f/4. (In fact, it is two stops bigger.)
A lens that has a large aperture (a small number) is referred to as a “fast” lens. In practice, numbers larger than f/4 are “fast” and smaller than f/8 are “slow”. It is likely that these terms relate back to the shutter speed: a “fast lens” lets you use a fast shutter speed.
Unfortunately, most cameras show the denominator of this fraction as the aperture value. But remember that the aperture called “2.8” is still bigger than the aperture called “4” (because it is really “1/2.8” versus “1/4”.
Next: What is a “stop”