A camera is basically a light tight box, and in order to expose an image, there must be a way of letting in a controlled amount of light.
An aperture is simply a hole that lets in light.
Adjusting the size of the aperture controls the amount of light that enters the camera during exposure.
The size of the aperture opening is measured in f-stops.
Starting with f/1.0 we have a series of numbers that represent full stops.