In music, that might be a bit of a melody that asserts itself periodically and kinda informs the rest of the song. It's equally interesting in design, where a theme — perhaps a geometric element — is sprinkled in tastefully that helps the gestalt. Ties the room together, as they say.
Anyway, if you're serious about getting better at design, his course is where it's at and opens up in mid-June. So, now's the time to think about it.