A statement of work is a formal document defining the specific deliverables, timeline, cost and terms of a project — more detailed and legally binding than a general proposal, and often incorporated into or attached to the main contract.
It typically spells out exactly what is and isn't included (number of concepts, revision rounds, specific file formats), which is precisely what prevents disagreements later about whether something was part of the original scope.
For any project of meaningful size, a clear SOW is worth insisting on — it protects both the client from unclear deliverables and the designer from open-ended scope creep.