When playing live you go up in ranks like in the military. While doing that you unlock weapons and perks by gaining XP. Ex. By using the MP5 you get 25 kills and you get bonus xp points and a perk for that weapon (Red dot site, Silencer, ACOG scope) Once you level a gun up to the Marksman 3 then you can't level it up anymore and you can move on to another gun. Once you reach level 56 you have the option to go "prestige" meaning all the perks and weapon powerups you have unlocked go back to zero and you have to start all over from level 0, however your symbol (aka prestige medal) changes. You will get a new one each time you do it. Once you hit level 10 prestige, then you are essentially done (until they release the new update from what I hear). Its totally by choice but then you also are frowned upon by the live community when someone hits level 56 and never prestige because essentially once you hit that level you should have most of your weapons powered up and all of your side perks unlocked. There are other elements and weapons you can unlock by powering up each class of weapons, ex leveling up all the assault rifles will give you the gold AK-47, all the shotguns give you the gold shotgun.