Ever wonder why sometimes an obvious foul is not called?

There is a clause in the FIFA laws of the game ("rules") that gives the Referee the discretion to allow play to continue even after a foul has been committed.  If the referee feels that by stopping play he would unfairly punish the fouled team, he can indicate to "play on" by reaching both arms toward the area of the fould and verbally yell, "play on".

For example, if the fouled team had a breakaway & might score even after having been fouled, he should allow play to continue. The idea is that the team which committed the foul should not gain an advantage as a result of the foul.