The generalization principle requires that the reasons for your action be consistent with the assumption that everyone with the same reasons acts the same way.
The reasons for your action are the conditions under which you would perform the action. They must be sufficient and necessary, meaning that you would perform the action whenever all the reason apply, and you would not perform it if any one reason failed to apply.
An act that satisfies the generalization principle is said to be generalizable or to pass the generalization test.
Your act passes the generalization test only if you can rationally believe that the action would still achieve its purpose if everyone with the same reasons acted the same way.
For a fuller explanation, watch the video Rational choice I (or read the transcript). It is part of an online ethics tutorial that consists of the following 30-minute sessions: