I read it in the text below:

Cannot is the correct form in almost all cases. The only (very rare) exception is in sentences like "You can do it, or you can not do it"—in other words, where the two words have their separate meanings ('are able not to'). In the normal sense ('not able to'), it is always one word. Anyone who tells you different is trying to get you red-penciled.

What does it mean? Is it a phrase or expression?