As you travel through the different areas in Deathloop, enemies will constantly be searching for you or on the lookout for you. Not to mention the ever-present threat of Julianna. So to even the odds, you will want to focus.
To focus in Deathloop, press the T key on mouse and keyboard. If you are playing on console, simply aim your crosshair at an enemy and hold down on the D-pad. Pressing the focus button briefly will tag them while holding it down longer will focus on them.
Related: What does Focus do in Deathloop?
Focusing on an enemy will reveal valuable information about them. You can focus on most enemies in Deathloop. The other option you have is to tag an enemy. Once you have done so, an icon will appear over their heads. You will then be able to track their movements and sneak around them much easier.
While you need to aim down your sights to mark an enemy, you have will have to stop aiming down sights to see the information that is revealed by focusing. Deathloop is constantly changing as you play so focusing is a key ability if you plan to escape the loop that Colt is trapped in and solve its puzzle.
For more information on Deathloop, check out What do you do with the Crank-Wheel in The Complex in Deathloop? and Should you listen to the floating text that appears in Deathloop? on Pro Game Guides.