Do singers actually know the meaning of their songs or do they just sing the songs that song writers write?

Do singers actually know the meaning of their songs or do they just sing the songs that song writers write? And do they actually write their own songs in the first place?? Shania Twain's song, "Ka-Ching" has a lot of true meanings in it, but I doubt she knew half the things she sang in that song... That's just one example though...