This function removes unused weight maps (maps with no weights assigned or only zero values assigned)
Note: In the operator Redo panel you find a few options for changing the behavior of this operator right after you called it (see below).
Hint: By default the operator is only applied to the active object. When you enable the crossed bones icon then the operator is applied to all selected objects.
In the operator Redo Panel you can set some options to remove only specific subsets of weight groups. See the option tool tips for more detailed information.