Motion sickness is caused by your inner ear and eyes sending conflicting information to your brain. Your brain then thinks you’re hallucinating, which means you might have ingested poison, and you need to expel it. So you get nauseous.
Imagine you’re in a closed room on a ship in rough seas. Your inner ear perceives you going up and down on the waves, which could be huge movements, but you are barely moving at all in relation to your surroundings (the room). That confuses your brain. If you’re on the deck and staring at the horizon, the tiny movements of your eyes tracking it as you move will match up more closely to your inner ear’s signals, which can help you not become sick.
Closing your eyes can also help, as it removes the conflict in signals.