Contra proferentem
against the one who put it forward
Where the phrase comes from
A rule of interpretation. If a term is genuinely ambiguous, it is read against the party who drafted it.
What lawyers are pointing at
The rule waits for a real ambiguity. It is not a tool for rewriting a clause that has a plain meaning you now dislike. Courts use it carefully where both sides drafted, or where a statute already tells the court how to read a consumer term. The practical lesson for the drafter is dull and useful: say which obligation, which deadline, and which remedy. Ambiguity is a cost the drafter pays.
The same shape in code
An error string that says 'invalid request' is ambiguous, and the service that wrote it bears the reading. Name the field and the constraint. A client who guesses wrong is not the author of the ambiguity.
A concrete picture
An insurance clause can be read as covering flood, or as excluding it, and both readings fit the grammar. The insurer drafted it. The narrower exclusion is the one the insurer has to live with.