Another way of saying it is up to five instances of itunes on different computers can be "authorized" to your itunes account (Apple ID). Once you hit the limit, no more can be authorized until some are deauthorized.
The best thing is to always deauthorize a computer before you cease to have access to it. Understanding people may forget to do so, Apple gives a way to clear all authorized computers, you can do this once per year. See
http://support.apple.com/kb/ht1420