Contractualism
There are two ways to think of contractualism: broad (general) and narrow (specific).  Broadly, contractualism is the idea that morality comes from agreements between people.  Keeping your promises is good and breaking them is bad.  Narrowly, contractualism is a set of ideas from Harvard philosopher T.M. Scanlon.  Scanlon wrote an essay, "What We Owe to Each Other."