Any body submerged in a fluid is subject to many small forces acting on each face. The sum of all the small forces is represented by a single force on each face. The visual only presents the forces exerted by a liquid on three sides of a cube submerged in it. The fluid exerts forces on each side, in a perpendicular direction to each one. If each side could be deformed, they would all buckle. If you want to see how a liquid exerts force on a submerged cylinder, click here.