What metric should I focus on when executing my workouts?

In order to execute workouts as they are intended, it is important to use the primary metric to guide your effort. The primary metric can be either power(lightning bolt), pace(clock), or heart rate(heart). Typically, bike workouts have a primary metric of power, and swims and runs have a primary metric of pace. Easy sessions typically have heart rate as the primary metric. 

To identify what the primary metric is for a session, look at your Sessions page for your upcoming workout and notice the colored icon below the title. The default icon is also highlighted at the bottom of the page showing your intensity zones.  You can click to see other zones by metric as well.  Your view will default to showing you the primary metric zones.