Prestige is a system that gives you special PvP bonuses, but resets your PvP levels. Upgrading prestige changes the color of your avatar frame. To upgrade prestige, you must first reach level 50 in PvP. Then head to your profile, and you should see some Roman Numerals. Press that, and a menu will show up showing you all of the bonuses you get with Prestige levels. Upgrading Prestige will reset your PvP level back to 1, and you will have to level up to get back all of your gear again, so make sure you want to upgrade your Prestige level. There are only seven Prestige levels, and it's to your advantage to complete each set.
Each prestige level has a bonus once you upgrade your prestige.
Prestige Level 0: none
Prestige Level 1: extra 10% DEF in PvP
Prestige Level 2: extra 10% DMG in PvP
Prestige Level 3: 1 extra PvP loadout
Prestige Level 4: 1 extra PvP perk
Prestige Level 5: extra 0.5 SPD in PvP
Prestige Level 6: extra 10% HP in PvP
Prestige Level 7: 1 extra PvP perk