Being able to read the gap is a very valuable skill as a Martial Artist. Gap is the distance between you and your opponent.Understanding that distance and how that distance favours you and your strengths is such an important part of your training.
As an example, if you are sparring a person who is a kicker, you may want to stay just far enough at a range that the opponent has to move in to kick you. The second the opponent initiates their advance and if you step into him, you have greatly changed the plan he has formulated on their attack.
Most people will either try to stand in place and block the kick or try to move back an equal distance in hopes to follow the kick in after it has reached it extension. Some people are so intimidated by big kicks coming at them that
they just want to avoid the kick as a whole and will move way back when they see it coming.