It is on the ship menu, the weird icon under your portrait.
No stars is unskilled, one star is novice, two stars is journeyman, three stars is expert, four stars is master, four stars with a large center star is legendary, and at least for captains there is a sixth rank that is a large blue star with five small green stars. Let's call it Legendary Master Captain for now .
Oh I finally know what meaning about that symbol. Help me a lot and thank you.