Also known as stoppage time, it is playing time beyond the designated period duration, at the discretion of the referee. This is based upon time stopped during regulation for injuries, out of bounds, and substitutions. Typically the amount of time is between two and four minutes.
One of the unique aspects about soccer is that time can be added back on to the game clock. This is known as added time or stoppage time. Added time is applied at the end of each half and is based on time that was wasted on the field, like due to things like player injuries or other major delays that take place on the field.
The match officials keeps track of all the time related to injuries and call it injury time. The officials are the only ones who dictate how much time, if any, is tacked on to the end of a half.
The fundamental idea is that during the match, there is time when soccer is not being played. Whether it be an injury, a set piece, a substitution or a throw in, there is time spent when the ball is not in play and the game is paused. In an effort to curtail time wasting, the head official is supposed to keep track of this time and add it to the end of a half. However, officials don't often accurately keep this time (as they are not supposed to confer together to determine this) and so, players still attempt to waste time even though some of it will be added on at the end.