Well, Frege's big thing (the big thing that didn't fall over, anyway) is a distinction between 'sense' and 'reference', where the 'sense' of a word is something like what we mean by it, and the reference of a word is the actual, real thing the word is about. He came up with this to explain why someone could know the meaning (in the sense of 'sense') of 'the evening star' and 'the morning star' without knowing that they're in fact the same thing (they have the same referent, i.e. Venus).

See: http://en.wikipedia.org/wiki/Sense_and_reference