It's when the black line is above the green bar.

This feature gets the name "training load" because it's trying to answer the same questions as the TSS system. This is one of those questions. It's just presented differently.

Which one works best for a person tends to depend on lifestyle. If cycling is the only exercise you get, the TSS system will probably be more precise for you - as long as your FTP is reasonably correct. If you run, swim, etc, then your power data only tells you anything about your cycling and the Garmin system works much better because it's holistic.