Units are used to subdivide a curriculum or class type. There must be at least one Unit defined for each curriculum.
When creating a Curriculum, for example "Group Piano" you can use Units to distinguish Beginner from "Year2" or "Level 3" etc. If you want to define materials that are associated with a class type, you can specify different materials for each of its units.
When you create a class, after selecting the curriculum for it, you will need to specify which unit that class uses.
Class types with no units (Music Together classes for example) are set up with one "dummy" unit usually named "~"