A Rigid Bicycle is neither equipped with a front suspension or a rear suspension. The advantage of not having any suspension systems equipped on a Bicycle is that you will get great responsiveness out of the pedals. No energy is being absorbed by the suspension so it is all going into the surface. This is good for uphill and flat terrains. The disadvantage then, is that a Rigid Bicycle will not be particularly comfortable to ride if the terrain is uneven and will be difficult to withstand when going through obstacles. Any sort of drops will be extremely punishing and large drops are almost impossible to land.
It would seem then that if you want to ride on a wide range of terrains and inclines (or declines) then it is necessary to either put up with a difficult and punishing experience half the time or buy two Bicycles. Of course, neither of these options seem attractive to either your body or your wallet. It is for this exact reason that the lockout fork was invented. With a lockout fork a Suspension Bicycle can be utilized when riding downhill/flat surfaces and on rough terrain. With the flip of a single switch the suspension can be made more rigid for riding on flatter and smoother surfaces.