The aperture is the gap in the centre of the blades of a lens. It controls HOW MUCH light enters the camera. When the aperture is adjusted it decreases and increases the amount of light entering the camera, by making the whole larger or smaller (a large or small aperture).

The aperture is measured in stops indicated by a fraction:
f/1.4 f/2.0 f/2.8 f/4.0 f/5.6 f/8 f/11 f/18 f/22 each aperture is half or double the amount of light of the preceding aperture. The smaller the number the bigger the aperture f/1.8 is a very LARGE aperture and lets in lots of light f/22 is a SMALL aperture and only lets in a small amount of light.