Once you've selected the areas of your site that the survey will fire on, you'll want to start describing the circumstances where someone could see the survey. One of these options is by site traffic.
Under "Who should be prompted to take this survey?" you'll see two options: Target a percentage of all site visitors, and Target a percentage of all visitors that match the criteria below.
The first option, if left at 100%, will target all traffic to the areas of your site your designated in the "Where should this survey appear?" section. If you use the drop-down, you can target just a portion of that traffic.
You'll only want to select the "Target a percentage of all visitors that match the criteria below" radio button if you are also using one of the targeting by behavior, technology, or advanced options.