It means raising a dead thread, bit like necromancy just with threads not zombies.
What counts as a 'dead thread' depends on both the thread and what your adding, a troubleshooting thread from 2 years ago is stupid to revive, but a still relevant guide to which your adding some new and relevant info to is a more gray area. If its a sticky then its fair game.
In general the test is, is what your adding relevant to that thread, or that topic in general. If the latter then its probably worth starting a new thread if there's none from the past month. (Assuming there's need for another, some topics are just done to death.)
Where the 'past month' threshold comes is a matter for much debate. Some people say a week of no posts and it should never be touched ever, others think months and its still cool.
