From the Office Open XML specification:

gridAfter (Grid Columns After Last Cell):

This element specifies the number of grid columns in the parent table's table grid which shall be left after the last cell in the table row. 

gridBefore (Grid Columns Before First Cell): 

This element specifies the number of grid columns in the parent table's table grid which must be skipped before the contents of this table row (its table cells) are added to the parent table. [This property is used to specify tables whose leading edge (left for left-to-right tables, right for right-to-left tables) does not start at the first grid column (the same shared edge)]