Aperature is the opening in the lens that allows light in, and it can be made larger or smaller. A critical attribute of a lens is how large that opening can become, which is called an f-stop.

The f-stops are broken out like so: F2.8 F4 F5.6 F8 F11 F16 F22 F32. Sliding along this scale (right to left) is called stepping up or stepping down. Each step results in a 50% light penetration difference.

Smaller aperatures result in larger depths of field. Closing the aperature by one f-stop gives you approximately 40% more depth of field.

A really basic photography resource for those wanting to know the fundamentals.