These are the various icons you'll see in your Unit Status circle throughout the game. Enthusiastic is the best one and you'd like to work towards / stay in that status as much as you can.
The game manual doesn't specify what should happen if you're in Humming status and do a vocal lesson (for example). It stands to reason that doing so might be another thing that could give a bump up to your danketsu power in addition to the lesson stat boost.