It means you're practicing "thread necromancy" - a fancy term for bring old threads to life. It's the primary sign of a noob - posting on old threads to bump their post count. Many boards will temp ban you if you do it too often, especially if you've been warned.
So unless you have something SUBSTANTIAL to add to an old thread (like in this thread, if you had a link to some source code for the game port requested), DON'T POST!! Leave it dead.