If you go to your own site often, chances are that you will want to exclude your visits from the stats so as not to inflate your results artificially. That is the purpose of the 'Exclude my own visits' setting.
If you click Yes, whenever you load one of your pages, 123Stat will recognize your browser and will disregard the visit. If anybody else comes to your site, the visit will be counted normally.
If you click No all visits will be counted, including yours.
Important Note : when you exclude yourself from the count, you actually tell 123Stat to exclude your browser. If you use several browsers (on the same computer or on different computers), you need to connect to 123Stat with each one of them, go to the Control Panel and set the exclusion.