The stock of a rifle is basically the parts that hold the barrel and firing mechanism. This includes the portion held against the shoulder when shooting. The firing of a bullet causes “recoil,” which is the backward push of the rifle as a bullet accelerates forward.
A “bump stock” is a replacement stock that uses the recoil to help press the trigger again, allowing repeated firing to happen faster than is possible with the shooter’s finger alone. In effect, it allows a semi-automatic rifle to repeatedly fire as fast as an automatic rifle. Because a semi-automatic rifle can only fire as fast as the shooter can pull the trigger, the maximum number of rounds per minute is between 60 and 180. The bump stock increases this number because manual trigger pulls are no longer necessary.
It appears that, even if outlawed, bump stocks may not be difficult to make at home. However, if they were illegal, it would prevent sales by legitimate gunshops as well as use of them at public firing and practice ranges. This video shows and describes the bump stock, bump firing and the difficulty in regulating them without the help of Congress. It’s well worth a few minutes to watch.