The Count Leaves column allows you to quickly see how many sub-items of a work item are at the last level of the hierarchy (they have no sub-items of their own). This is particularly useful when you have organized work items using Group generators or Flex Items and you need to count just the work items themselves.
Notice in the example above that the Big Project group has 5 sub-items, but only 4 leaves. The Count Leaves column allows you to omit group items (in this case, the sprint groups) and look just at the number of work items in the last level of the structure's hierarchy.
See also: Count Sub-Items Column