I can do vertically submerged fluid force problems. One portion of my book says:

If a flat surface is immersed so that it makes an angle of with the vertical, then the fluid force on the surface is given by:

Can someone explain exactly what the sec(theta) is giving me; how exactly this equation changing the original equation of a vertically submerged surface. I never took trig., so please dumb it down in that area

*EDIT* If it helps, just made a diagram. Is this the correct angle in the equation?