A contract between the dominant and the submissive. It is usually found in a master/slave dynamic. Other parties for e.g. Dom and Sub who „just“ play together but do not live in a 24/7 kind of relationship can have a written contract as well tho.
What a contract contains is individual. Most commonly it is rules and how the submissive part has to behave in order to please their dominant other half.
read more under slave contracts