Pressure is defined as the following Pressure = force / area. So what this means is pressure is how much force is exerted over a area of given size. For examle a concrete slab is on the floor this slab has a width of 2m and a length of 2 m its area is 2 x 2 which gives 2m Squared. Its force it exerts is 1000 newtons ok what the hell does this mean? if you place this in the formula you will get pressure = 1000/4 Pressure = 250 newtons per square metre. I hope this answers you question on what pressure is.