Right now, as you are reading this, there are tons of air pushing down on you. That is air pressure. As you might know, the Earth is covered with a blanket of gases. That is called the atmosphere. The air from the atmosphere takes up space, so it has mass (weight). The more atmosphere above you, the more the air pressure. There is actually 14.7 pounds per square inch of your body pushing down on you right now!! That means that your whole body is having about 7 tons of air pushing down on you! You can’t really feel the air pressure because your body is pushing outward, the opposite direction of the air pressure.
Water pressure is how much water is pushing down on an object. Water pressure is measured by weight. That is why they do not allow toddlers in the deep end of the swimming pool. It is because their bodies can’t handle the water pressure pushing down. Submarines have to be built to withstand the pressure because they might be crushed if they don’t have a good structure.