The Count Leaves column allows you to quickly see how many sub-items of an issue are at the last level of the hierarchy (they have no sub-items of their own). This is particularly useful when you have organized issues using Group generators or folders and you need a count of just the issues themselves.
Notice in the example above that HP Sprint 4 has 13 sub-items, but only 9 leaves. The Count Leaves column allows you to omit group items (in this case, the Assignee groups) and look just at the number of issues in the last level of the structure's hierarchy.
See also: Count Sub-Items Column
You can toggle between Count Sub-Items and Count Leaves. Open the column configuration, click the settings icon and check/uncheck Leaves Only.