This is calculated as how often does a keyword appear in a website title or description. You do not want to go overboard with frequency, however, since on some engines if you repeat a word too many times, you are be penalized for "spamming" or keyword stuffing.
In general though, repeat your keyword in the document as many times as you can get away with, and up to 3-7 times in your list of metatags.
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