i believe that would be the shared ground between the headphones or mic ports.
you will need to connect it and see which one it is. if it is a port you won't use then its not a problem.
edit: if it causing a problem find out where it should go and try using something small like a paper clip to "re-insert" it back in the plug. it only need to make a connection, then secure it with electrician's tape so it doesn't come out again.