The Assign Keyword function takes all transactions with that same keyword you see noted to the right of the check box and categorizes them all the same way. This would include all past and future transactions. Please note that in some cases, completely different transactions may have the same "keyword" noted, so it could potentially improperly categorize any given transaction.
A good example would be if "deposit" is the listed keyword. You may not want to use the Assign Keyword function here because it will categorize all the transactions with "deposit" as a keyword in the same way. Where it could have been your Direct Deposit for your regular Paycheck, or a cash deposit you made to cover a bill—you most likely wouldn't want them categorized in the same way.