Mastery Levels are useful for quickly visualizing student performance instead of having to scan individual accuracy percentages. It saves you time when analyzing results and provides a common language around student performance.
In order, the possible levels and their Raw Accuracy cutoffs are:
Mastery - 90%+
Excelling - 80-89%
Proficient - 70-79%
Passing - 55-69%
Struggling - 0-54%
These are set by default in Albert and can't be customized.
A few places you can find Mastery Levels are:
On class dashboards:

On assignment and assessment data:
