Fluids exert pressure on all bodies immersed in them and on the walls of the container that holds them.
E.g. A balloon expands when we blow air into it. The air inside the balloon exerts a pressure on the inner wall of the balloon. If we blow in too much air, and the material of the balloon is not capable of expanding further, increasing the pressure inside can cause the wall of the balloon to break at one or more points. This is why a balloon bursts when too much air is blown into it.
