A stovepipe is a failure to properly extract after firing. The casing sticks on the bolt head or extractor, and usually ends up partially exiting the ejector port, with the open end facing straight up.
Since the freshly fired brass is usually still smoking, it looks like a stove pipe sticking out the port.
I can be caused by everything from bolt and extractor problems, to a sloppy/sticky action or weak gas system. Low power ammunition can also cause this, as there may not be enough gas created to fully compress the action.