This quote comes from a post by Derek Sivers, although the origin appears to be from a joke where one fish asks another fish "how's the water?" The other fish replies, "what the hell is water?"
The point: it's easy to forget that what surrounds you is only normal because it's what you know. To others, those surroundings might seem pretty strange.
Sivers points to this as a cultural phenomenon, but it's also really applicable to your work environment. Worlds converge at the office, and not everyone is the same. When someone does something strange, or reacts strangely to something you do, it might simply be because you're thinking in different terms. Staying aware of the "water" you'd otherwise neglect is a good way to avoid miscommunication.
Fish don't know they're in water [Derek Sivers]