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Grouping keywords that mean the same thing into one cluster, so one page serves all of them rather than a page per phrasing.
Search engines match meaning, not strings. "How to write a project plan" and "project planning steps" are one question, and one page should answer it.
The reliable method is to compare what ranks. If two queries return substantially the same pages, the engine treats them as the same question and they belong together.
Grouping by word similarity alone is unreliable — closely worded queries sometimes have different intents, and differently worded ones often do not.
Compare what ranks. Substantially the same results means the same question.
Yes. More than one is how cannibalisation starts.
Update it. That is usually a better investment than a new page.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.