There's a principle in Anglo-American contract law under which ambiguous contract language is to be construed against the drafter. In other words, if one party drafted (wrote) the contract, then any term whose meaning isn't 100% clear will be interpreted to the detriment of that party, and in favor of the other party (the one that did not draft the contract).
You include a "fair meaning" clause to change that rule: to ensure that if the parties later end up litigating the contract, the court will NOT apply that rule; instead, regardless of who wrote the contract, the court will interpret any ambiguous terms according to their "fair meaning" (whatever the court can best determine, based on the language itself and any extrinsic evidence that may come in, is the proper meaning of that term).
I disagreed with PhB's answer because a fair meaning clause does NOT mean that the court will interpret every term in a way that's fair/equitable to the parties. It's fair to the language itself, not fair to the parties. Juste is best.
Basic explanation:
https://www.allbusiness.com/ambiguities-clause-in-contracts-... Sample clauses:
https://www.lawinsider.com/clause/fair-meaning