Basically, it is a set screw that is inside an escutcheon set into the forearm of a rifle so that it can be screwed into the barrel channel until it applies pressure to the barrel. By applying pressure to the barrel you change the harmonics and hopefully "tune" the barrel to the load. It is most often used with rimfires as you cannot adjust the load, so you tune the rifle to the load you are using.
The biggest problem is that if you don't put it in the optimum location down the length of the barrel, you may not be putting your pressure point in the best place to tune. Seems like a few years ago somebody was making a stock with a channel down the length of the forearm that had a captive screw mechanism and a block of Delrin that would move fore and aft until you found the sweet spot of the barrel for tuning. I don't believe they sold very well as you could accomplish the same thing by floating your barrel and monkeying with the load - except for rimfires of course.
More info than you wanted probably but I am on my 4th cup of coffee.