An impression happens every time someone sees your link or ad, whether they click on it or not. The ratio of impressions to clicks tells you something about the efficacy of your ad or link. Counting impressions is fairly easy by counting the number of time an image is displayed on a page. Amazon and Commission Junction use tiny little 1 x 1 pixel images with text links, so that things don't get slowed down much, but any image will do the job.
If you have access to a server log, that is one crude way of counting the number of times an image file is accessed, or a simple PHP script will also work. But, what if you don't have access to a server, don't know a log from a hole in the ground, or never heard of PHP?
There's a simple trick you can use.