The coffin corner is a general term for an area extremely close to the opposing team's goal line. It refers to a punt that crosses over the sideline and goes out of bounds just before it reaches the goal line. A good punt of this kind puts the receiving team deep in the green zone, giving the opposing team very poor field position that makes scoring much more difficult.
In today's games, the coffin corner punt is not used all that often for a number of reasons. For example, it is a difficult punt to execute. Asking a punter to consistently make that kick over the course of an entire game can be a lot to ask when it would be much safer to kick the ball inbounds with a higher chance of it making it further down the field.
Additionally, as defenses become faster, punters have even less time to line up their kick, making it that much harder to aim the ball at the corner of the sideline and goal line. Moreover, keeping the ball in play does give the receiving team a chance to run the ball back, but it also creates an opportunity to fumble. This would not be the case if the ball does not stay inbounds.