I used to get carsick as a kid. VR can do this to you too — the reasons for this were discussed earlier while introducing VR, but this is a very important topic, so it bears repeating.
If you move a point of view, independently of what the user is doing (user agency), the brain knows that it didn't move. Yet, the brain also sees the world moving through your (VR) eyes. The brain then relies on a very ancient, important survival characteristic—you will think you've been poisoned.
When you've been poisoned, your body is very good at emesis. In less clinical terms, you throw up. Your body figures that something is trying to kill you, so it just gets rid of whatever is in your stomach as a panic reaction.
So, how do you move around in VR? How to enable VR locomotion without making people loco?