When you give an object, such as a user, rights to another object, such as a container, you make a trustee assignment. That user then becomes a trustee of that container. Any object with a trustee assignment to another object is a trustee of that object.
Each object contains a list of objects that have trustee assignments to it, called a trustee list. This list tells who can access that object. An object's trustee list is stored in its Access Control List (ACL) property.