In short
The percentage of road-test candidates who pass at a given DMV center or state-wide.
Road test pass rates vary by state and DMV center. Most states publish them at the state level (typically 50–80%); individual DMV centers can range from about 30% to over 85%.
Pass rates vary mostly because of the local road environment (busy urban DMVs are harder than quiet rural ones), not examiner leniency.