You need to look at the description, too

A normal person would have the following interpretation: typically 3 means "meet all expectations",  and 4 means "meeting all expectations and occasionally exceeding expectations".  This would make 5 mean "exceeding almost all expectations" and if you get 5 two years in a row, it's time for promotion.

Of course there are a lot of abnormal organziations and abnormal bosses:)