Waterboarding is a really scary thing that can happen to someone. Imagine you are swimming and you accidentally breathe in a bunch of water. You might cough a lot and feel like you are choking. Well, waterboarding is kind of like that, but much much worse.
When someone is waterboarded, they are tied down so they can't move. Then, someone pours water over their face so that they can't breathe properly. It feels like you are drowning and can be very very scary. The person doing the waterboarding might stop for a few minutes to let the person catch their breath, but then they start again.
Waterboarding is sometimes used as a way to make someone talk and give information they might not want to give. It is not a nice thing to do and can cause a lot of harm to the person being waterboarded. It is not a safe or kind thing to do and is not something that should be done to anyone.