The counter allows you to keep track of how many visitors have been to your site. This is not a very scientific method, but at any rate it gives you a fairly decent idea of how many people have viewed your page.
When you click on the Counter link you will see numerous configuration options available. The best way to explain each of the options, is to have you play with all of them. You can click the Preview button to see what the counter might look like. Your best bet is to change these options until you get the look that you like best. You can then click the Make HTML button and the HTML code will be displayed to paste into your HTML document.
You will notice you can make either a Counter, a Clock, or display the Date with this script. You can choose any of these, or you can make multiple entries. Be sure that you name each Counter, Clock, or Date something different where it says Counter Name:. Generally it is a good idea to name your counter your username. If you are going to use more than one counter or combinations of counter, clock or date, you should prefix the name with your account username. That is prefix it with the username you use to log into the Control Panel. For example, if my username was ams and I had already created a counter named ams and I decided I wanted a clock as well, I might call my clock ams_clock. This will help to insure that everybody's counter, clock, and date will remain their own.