In fighting games (such as Injustice: Gods Among Us), commentators of a match describe moves by how much that they are plus or minus on block. What does it mean?
When commentators of a fighting games are talking about + or - on block they're specifically talking about frames. To understand this concept, you have to come to terms that fighting games operate at a predefined rate. The rate of which a fighting game operates is often declared in frames per second (fps), literally how many frames or timesteps occur per second. Most fighting games today operate at 60fps so in other words within each second you can split the game into 60 separate states.
Coming back to frames it's best to explain it with an example. So in Tekken most jabs offer +1 frame advantage on block. This means that if the attacker's jab is blocked the opponent CANNOT perform any action in the next frame. That is, the defender cannot do anything for the next 1/60th of a second. So if both players are mashing jab after the initial jab is blocked, the defending player will always lose because he is in a -1 frame disadvantage or his jab will come out 1/60th of a second after the attacker (who has a +1 frame advantage).
Conversely in a -frame situation, for example getting an uppercut blocked in Street Fighter. The attacker cannot perform any action while recovering from the move, hence he is both in a frame disadvantage and punishable. So it can be stated that if a player is in -n frame disadvantage, he is punishable by moves which can connect in <=|n| frames.
This is of course just the tip of the iceberg when discussing frames, one could go into details regarding frame traps and other intricacies. You can get more information regarding frames from dedicated fight game forums, I highly recommend UltraChenTV's episode on frames