To get a better idea of what it means to express a proposition, compare this to something that "does not" express a proposition. Suppose someone minding a convenience store sees a thief pick up a candy bar and run. The storekeeper manages to exclaim, "Hey!" In this case, "Hey!" does not express a proposition. Among the things that the exclamation does not express are, "that's a thief there"; "thieving is wrong"; "please stop that thief"; or "that thief really annoys me." The storekeeper isn't saying anything that can be true or false. So it is not a "proposition" that the storekeeper is expressing. Perhaps it is an "emotional state" that is being expressed. The storekeeper is surprised and angered, and expresses those feelings by saying, "Hey!"