According to Wikipedia:
In the form of a screw jack it is commonly used to lift moderately heavy weights, such as vehicles. More commonly it is used as an adjustable support for heavy loads, such as the foundations of houses, or large vehicles. These can support a heavy load, but not lift it.
Insights into its mechanism is greatly appreciated .
A jackscrew is really an inclined plane wrapped around a rod that is then turned by a lever. These two simple machines dramatically reduce the level of force needed to lift items, allowing people to lift very heavy objects, like cars.
To illustrate how the two simple machines involved work, imagine you have a really heavy package to bring into your house. Now, you house has a short staircase to your door, and it also has a gradual ramp to the front door. You'd probably use the ramp, right? The reason is that, while you do the same amount of work (the ramp still rises the same height), it is done more gradually when you use the ramp - the force is spread out over a period of time.
A lever has several different examples, but the best, I think, is a pair of pliers. You don't have to squeeze to hard to close pliers on something, but it exerts a whole lot of force. That's because the handles are longer than the clamps - you put in a certain amount of force, and it is exerted over a shorter distance.
Combine these two now, and it makes it much easier to exert enough force to lift a car. Hope this helps!