The mechanical advantage of an inclined plane is defined as the length of the ramp divided by the height at the tall side. For example, suppose a pickup truck bed is two feet off the ground. You place a board eight feet long on the bed, to load a motorcycle. The mechanical advantage is 8 feet divided by 2 feet, or a factor of four. In other words, you will have to exert four times less energy to load the motorcycle, then if you were to just pick it up and load it.
Work is defined as force times distance. In the motorcycle example, suppose the motorcycle weighs 200 lbs. To lift 200 lbs. 2 feet straight up to the truck bed requires 400 pounds of work. The formula for work and mechanical advantage is complex but the end result is only 200 lbs. of force is needed to do 400 lbs. of work, due to the mechanical advantage.
A screw thread is a form of an inclined plane. This being the case, it has a mechanical advantage. The mechanical advantage is defined as the circumference divided by the pitch. The pitch is how steep the screw threads are. To see a screw as an inclined plane, take a paper tube and wrap a inclined plane or triangle cut out of paper around it, starting with the high or ramped side.
Notice it forms a spiral around the tube. This is a screw thread, which is an inclined plane wrapped around a cylinder. The calculated actual mechanical advantage is complex, but the formula itself turns out to be the same (length divided by height), since the length translates to the circumference, and the height translates to the pitch.