A pixel is, simply put, dots that make up graphics on your screen.
What 72x72 pixels means is the size of your graphic. An easy way to resize your graphic is to load it up in microsoft paint or a *cough*better*cough* graphics program and use a resize or stretch function to set it down to 72x72 pixels so you can use it as your avatar.
If you open up the picture in Paint, go to the Image menu at the top of the screen then click on Attributes (or press CTRL+E) and it'll show you how many pixels are in your picture. Then either adjust the attributes there or use the Stretch/Skew option (or press CTRL+W) under the Image menu.