Climbing on sight means that a climber is attempting a new route that they have never seen nor attempted before, nor have they seen anyone else attempt previously.

The trick in climbing a difficult route to the top in the first try is to read the route. Starting from the bottom, the climber will decide which hand will go to which grip. This way, the climber can identify the key movement as well as points for resting. With that extra information, the climber knows which parts of the route he needs to do reasonably quickly, where he can recuperate during the climb.