You'll lead either the British or German forces as you try to overcome the steadily approaching opposition. It's both a battle for land and morale as you can win by either moving to the far right of the board or just beating up on your foes until they can't take it any more.
You'll decide which unit types you want to send out when, as well as the best time for them to move from trench to trench. Also at your disposal is a wide array of different big weapons like mortars and artillery, which you can rarely deploy without making sure your forces are far, far away from the target area.
We haven't completeld Warfare 1917, so we can't quite put it on the level with Last Stand yet. But if the early bits we've played are any indication, you might want to free up a couple of hours before deciding to give Warfare a try.