It means it has cured of pokerus. It's a virus that a Pokemon gets that doubles the EVS you get and it only lasts for a short amount of time. It's not a bad thing but yet the complete opposite. If you'd cured of pokerus (like your cinncinno) then you cant get it again. Pokerus can spread to Pokemons in your party that haven't had it though. If you put a Pokemon in the pc with pokerus the time limit left for the pokerus duration stops. And then resums when its out.
Source-EXPERIENCE
