The main function of a follow focus unit is to give you a more intuitive way of pulling focus. The way it does this is it moves the focus mechanism to the side of the camera so that your hand or your camera assistant's hand doesn't have to be in front of the camera, but rather to the side. It's a simple gear mechanism, but there is a decent variation in quality between cheaper and more expensive units and we'll go through a few things that you should look out for.
The biggest concern with one of these units is how much play, or backlash, the gear mechanism has. In layman terms that is just saying that when you turn the knob, does the lens turn exactly at the same time the knob does, or is there a little movement before the lens starts to move. you can test out how much play a follow focus unit has by holding the gear end with one hand, and wiggling the focus knob with your other hand. A perfect unit should have no wiggle, but many cheaper units will have some wiggle. A little is fine, but less is better.
Most units feature a marking disc, a white disc which you can use a dry erase marker on so that you make repeatable focus pulls. You will typically put tape on the floor to show the actor where they should be at any point in time, and you label those floor markers to match up with marks you make on the follow focus's marking disc. Makes things peachy.
A few other details to look out for: Some units are Dampened, which means that if you had the unit not conneted to the lens, and you spun the focus knob, it won't spin freely, it's dampened to improve the smoothness of the pull and improve accuracy. It's a good thing, so try and be sure to get a unit that is in fact dampened. Another thing is height adjustability, different lenses are different sizes and if you don't have standardized lens gear sizes then you might want to get a unit which can raise up or down to match up to your various lenses.