Mechanical advantage is expressed mathematically as a ratio or fraction. The top number of the fraction is the force the machine puts out to perform the work you're trying to do. This top number is therefore called the output. The bottom number of the fraction is the force you have to put into the machine to make it do the work, and we call this number the input.

Give me an example!

Have you ever watched someone lift a car with a bumper jack? Obviously, a normal human can't lift a 1,000-pound vehicle off the ground, so you need the mechanical advantage of a jack. If the weight of the vehicle (the output) is 1,000 pounds and the weight of the jack handle (the input) is seven pounds, then the mechanical advantage provided by the jack is 1,000/7, or "one thousand to seven."