When you are in dead eye mode, depending on what weapon(s) you are using, you mark enemies (One, or several) by touching them with the "crosshaair", press left mouse button to execute your directive.
Edit: Just discovered; When you advance far enough you get to manually mark your enemies with "Q", when you are in dead eye mode, so you don't accidentally mark everybody on their thighs or shoot their horse or whatever all the time.