It means that a sailor has been correctly classed at the level when the new classing became available.

Example: Sailor => lvl 12 (KM) Support sailor => lvl 29 2nd engineer => lvl 52 1st engineer => lvl 65 Chief engineer

His primary ability is at a max and, except for pilots (who have diminishing crew growth with subsequent classing), his crew number also.

Do a quick search in the forums. There is heaps of info on classing/delayed classing, which sailors to do it for and the impact.