It is a very important way to see how a future battle will unfold.
One can access the combat preview by selecting a sub headed for an enemy outpost and clicking the magnifying glass icon. A green dot in the magnifying glass signifies a victory, while a red dot signifies defeat. A faster way of telling the outcome of a battle is a red or green blinking dot on the tip of the sub. Green indicates future victory, red indicates defeat.
Once upon clicking the combat preview, a pop-up box will display your drillers and specialists, and enemy drillers and specialists, as well as any shield for the defending player.
On the bottom, one can find the calculated end result with all specialist abilities accounted for. Clicking "OK" will close the combat preview.