This means that the code trees conflict in such a way that git cannot determine how to merge them safely, and leaves that up to you. This can be very common when mixing code trees, and fixing it might be very hard or very easy, depending on how much the code from the different trees mix with each other. In this case, it is fairly easy:
#include "GMTicketMgr.h"
#include "Util.h"
<<<<<<< HEAD:src/game/World.cpp
#include "OutdoorPvPMgr.h"
=======
#include "AuctionHouseBot.h"
>>>>>>> 8be8d5a61ae512b310e83ed62152ed80584f7f05:src/game/World.cpp
INSTANTIATE_SINGLETON_1( World );
This is taken from the file referenced in the error message. When git is unable to merge changes, it leaves the code from the previous version between <<<<<<< HEAD:Path and =======, and the code to be merged in between ======= and >>>>>>> (commit id:Path). Here, the programmer has to determine whether to keep the old code, the new code, or perhaps both. If you consider what codebases you have merged, you should be able to come up with the answer
When done, remove the lines introduced by git ( <<</>>>/=== ) and commit. Now you should be ready to compile, or even merge in additional branches!
Good luck!