Also called: histogram_bounds, equi-height histogram
In plain English
For the values that are not on the most-common list, PostgreSQL splits them into buckets that each hold about the same number of rows, and remembers the boundary value between buckets. That structure is an equi-depth histogram.
Why it matters
It lets the planner estimate range queries such as amount > 100 by counting how many buckets fall in range. Because every bucket holds roughly equal rows, even skewed data is described fairly. A bigger statistics target means more buckets and finer range estimates.