The Mask modifier allows vertices of an object to be hidden dynamically based on vertex groups.
The Mask Modifier can hide parts of a mesh based on two different modes, selectable from this select menu.
Normally, vertices belonging to the selected vertex group (or group associated with the active pose bone) will be shown. The Invert toggle allows you to reverse this behavior, instead only showing vertices which do not belong to the vertex group.
Hides vertices with weights less than or equal to this value.