The farm pump got its name because of the frequent usage on farms in former times. The key difference with a normal hand pump is the location of the piston. In a farm pump the piston is above ground level, just below the crank. The tube which is in the water has a foot valve at the end.

When the pump is not in use the tube with water deflates slowly because the piston and de foot valve aren’t entirely watertight. If the tube is empty, it won’t start up again just because the piston moves back and forth in air. This can be solved by filling the pump with water from above. Once the piston reach the water the pump will work again. This operation is called the “start” or “prime” of the pump.