A stovepipe, or a stovepipe jam, is a failure to eject
jam where the spent case gets lodged in the ejection port
between the breech and the barrel.
It comes from the fact that the firearm that suffers a stovepipe jam very much resembles a stovepipe.
Picture a semiauto pistol with the slide halfway forward and where the spent brass should be ejecting there is one lodged with the open end outwards, probably smoking a bit too after just being fired, you get the picture.
To clear this kind of jam just perform a tap, rack, bang! and you are good to go!
Stovepiping is a sign that your recoil spring might be too heavy for the loads you are using or more likely that your loads are too light for the gun you are using. Adjust accordingly.
You can also install an extended ejector, if it is possible for the model of your firearm, to ensure positive ejection of spent brass.
karfung sneakily picks up your spent brass and runs away with them. heehee.