Just to clarify; there are 50 levels to unlock on-line on Black Ops. These are dependent on your current experience points (xp), the higher your xp the higher your rank. When you reach level 50 you get the option to start all over again, a process called prestiging. There a further 15 levels of prestige, each is assigned a single symbol. Therefore a level 40, 2nd prestige will have the same icon as a level 10, 2nd prestige. I hope this makes it a bit clearer.
PS4 & PC - Currently playing WW2, Stick of Truth & La Noire