From a spiritual perspective floating in dreams is often a precursor to out of body experiences. In lucid dreaming we can learn to control flight, and to astral travel (deliberately change consciousness).
Psychological / emotional perspective: Because we are not taking charge of our own direction, we are being indecisive and perhaps need to think more carefully about our actions and involvements with other people.
Material aspects: Floating in a dream was considered by freud to be connected with sexuality, but is probably much more to do with the inherent need for freedom. Generally we are opening to power beyond our conscious self, when we are carried along apparently without our consent. We are in a state of extreme relaxation and are simply allowing events to carry us along.