Basically the larger the aperture the smaller the depth of field. The apeture value is a fraction of the lens diameter. So f2.8 is a large aperture and f22 is small.
For example:-
Setting you lens to f2.8 (larger apeture, the number is a fraction remember) will give u a smaller depth of field and the closer you get to an object the more obvious this small depth of field will become.
On the other end of the scale if you set the lens to f22 you will have a massive DOF. So on a landscape you would probably have sharp focus all the way to the horizon.