Each set in a MySwimPro workout can be assigned a Zone: a measure of intensity of effort on the set.
There are seven possible zones a set can have:
You can see what Zone the set should be swum with by looking at the color on the left side of a set. If you swipe right on the set, you will see a description of the Zone. The Zone will also appear on a workout loaded to a compatible wearable device, so you don't have to remember how hard to go during a set!
See also: What Are Swimming Training Zones?