The idea is that the meaning of an utterance isn't just one thing; it's kind of the overlap between two distinct propositions: the sense (that is, the concept or idea by which we find the referent), and the referent, the actual entity to which it refers. The standard textbook example is the word "water"; the sense of "water" would basically correspond to a descriptive, conceptually encoded "water"-iness, and the referent would be the substance itself. Basically, you've got your ideas about water, you've got the abstract entity you recognize or impute as a member of the reference class (the substance H2O considered in abstract, or a pond, or a glass of clear odorless transparent liquid on the table in front of you), and both are relevant to determining the actual semantic content.