Head tilt is exactly what it sounds like - the bunny's head becomes tilted to one side. It can be the result of various things, from physical injuries (such as Apple (JadeIcing's newest bun), EC, ear infections or other illnesses in bunnies.
It can make bunnies feel dizzy, affect their balance and their movement. However, causes behind head tilt can be fatal, and tilt that is not the direct result of physical injury is likely the symptom of something else.