Adding competitors makes it possible for you to compare the rankings for the keywords you have set with your competing websites. The competitors tab can be found when you have entered the Dashboard of the website you want to add competitors to.
When in the competitors tab simply enter the website of your competitor and press add.
Now you are able to track the traffic your competitors are receiving from your keywords as well as how they rank for each keyword.
It's possible to have up to ten competitors for each website, just add them the same way you added your first competitor.