It is the same thing as sea sickness and car sickness. But you have way more control over when you start and stop. So you can either go with dramamine or you can go with controlling the variables to train your brain that it doesn't need to protect you from "the poison berries". Motion sickness is caused by your eyes and inner-ears disagreeing with what is happening. Which for most of human history meant you were hallucinating from eating something poisonous and vomiting would be a good way to save your life.

But, the brain can be trained to lessen and even completely forego this response. If you immediately stop as soon as you have the first minor symptoms, usually warm face, and then wait an hour or so and go back in. You'll steadily increase the amount of time you can play before feeling symptoms. Your brain will subconsciously reinforce that it doesn't need to protect you, whatever is going on isn't life threatening. But the opposite is also true, if it keeps going far enough that you get most of the symptoms or even do vomit, it'll reinforce to your brain that it is indeed saving your life and the response will get faster and faster.

Dramamine can artificially slow down the response and buy you way more time to start with, which would make training it away even easier, and it's usually a very important part of training it away for sea sickness since you generally can't immediately stop being on a boat, and helpful for car sickness if you can't just have the car pull over for an hour when you feel it coming on.

There are also usually settings in most VR experiences to reduce how much they might trigger that response. For racing games, a "lock to horizon" option can really help. There will still be some milder triggers, but getting rid of that one can buy you a lot of time.. For other games, avoiding movement that isn't coming from your body in the real world pretty much eliminates VR sickness causes. But if the game really needs artificial movement, there are settings like vignetting and having more static graphic elements added to focus on during movement.

Eventually, with successful training, you won't need the comfort features. You'll be able to play any game for any amount of time.