Expands the region of the currently selected components outwards in all directions from the currently selected component on the polygonal mesh. The expanded selection is a boundary type selection that is dependent on the components in the original selection.
For example, if you have a polygon mesh comprised of four-sided polygons (quads) with one interior vertex selected and then choose Select > Grow Selection Region, the eight vertices that immediately surround the selected vertex are selected on the mesh (as these vertices form a boundary around the selected vertex). As you continue to grow a polygon selection the region of selection expands outwards.
Many of the polygon selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.