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 not psychological causes or motivations, but the reasons that you consciously take to justify taking 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 example, if you tell a lie simply because it is convenient to deceive someone, your reasons are: your lie will deceive someone, and the deception is convenient. This is not generalizable, because you cannot rationally believe that your lie would deceive anyone if people always lied when deception is convenient. People would assume you are lying like everybody else.
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:
- Business Ethics Course, Session 1: Why ethics? Transcript with slides.
- Business Ethics Course, Session 2: Rational choice I. Transcript with slides.
- Business Ethics Course, Session 3: Rational choice II. Transcript with slides.
- Business Ethics Course, Session 4: Daily life case studies. Transcript with slides.
- Business Ethics Course, Session 5: Business case studies I. Transcript with slides.
- Business Ethics Course, Session 6: Business case studies II. Transcript with slides.
Ethical Decisions 
I know that with the term “action” the entire process of arriving at the actual action is meant, but I believe it makes sense to spell it out explicitly that the action to be taken can, e.g., mean to cast a die and let randomness decide which action is carried out. Or, to give another example, the action taken may be to change parts or all of the decision-making process.
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