The amount of water being pushed aside. When the weight of an object equals the amount of water it pushes aside, this will make the abject float. The part of the submarine that holds water and air to make the submarine sink or float.
Subs can float on top of the water because the ballast tanks are filled with compressed air.
Since air weighs less than water, the sub displaces an amount of water that is equal to its weight. If you are in water and you take a deep breath you will float to the surface. When you let the air out of your lungs you will start to sink. To get the subs to sink under the water, the ballast tanks are filled with water. The sub weighs more, and the submarine cannot displace enough water to support the extra weight added to the sub.