As the missile moves, it can determine the amount of deviation in determining it's location by referring to its sensor data while moving through space. And it uses this deviation to course correct. From course correcting, you can assume that the missile is actually where you want it to be along its path. If upon course correcting, the missile is still off course, the missile has encountered a "variation" such as wind speed pushing the missile off course. So long as the variation is not significant, no changes are necessary to its course. If the changes are significant, however, because we know information such as the speed of the missile and what direction it was moving, we can be reasonably sure of its position based on this previous information within a certain margin despite the variation in actual position with expected position. This difference in expected position and actual position is called error.