The producers at Treyarch have announced that they are rewarding their loyal customers with bonus items in Call of Duty Black Ops 3 for their continued support. If you have reached a specific level and achieved a certain result in other Call of Duty games along with linking all of you accounts (which can be done here) you are eligible for the rewards offered in the loyalty program.
If you have reached level 10 or higher in Black Ops 2, Call of Duty Ghosts, or Call of Duty Advanced Warfare then you are eligible for the first level of prizes. If you reach level 30 or higher in any of the above games than you are eligible for the second level of prizes. If you preorder then you will receive the last level of prizes.
The program finishes on November 5th 2015 and is only available for people on the PC or a next generation console.
You can read all about the program from the official release here.