The Fresh Start option becomes available after reaching Prestige in Call of Duty: Black Ops 4.
Fresh Start completely resets all of your Multiplayer progress and everything that you've unlocked, including:
You will, however, keep all items earned through Operations and Special Orders and your Purchased Items.
After choosing the Fresh Start option, you will start over at Level 1 and everything in the game will be locked as if you have never played a Public Match in Multiplayer. You will start at the beginning as though you are playing the game for the very first time.
Permanent unlock tokens — including tokens that come with pre-orders of Call of Duty: Modern Warfare and tokens earned for reaching the maximum rank in the Call of Duty: Black Ops 4 Multiplayer Beta — are not refunded with Fresh Start.
Please note: Proceed with caution! Fresh Starts are final and cannot be reversed. Once this option is chosen, all data is deleted and cannot be recovered.