Also called: MCV list, most_common_vals
In plain English
When PostgreSQL studies a column with ANALYZE, it records a short list of the values that appear most often, together with how frequent each one is. That is the Most Common Values (MCV) list.
Why it matters
The planner uses the MCV list to estimate how many rows a WHERE col = ... filter will match. If your value is on the list, the estimate is exact. If a hot value is missing because the list is too short, the planner guesses badly and can choose a slow plan. Raising the statistics target makes the list longer.