Hi Esraa, the meaning is "to go to bed".

it means to go to bed. I doubt many native speakers would think much about the real meaning. To me, it could be the act of sitting on the bed, then 'turning / rolling in' to go from sitting to lying. Or it could be related to the bedsheets, which are 'turned down', a very specific term for folding down one corner of your bed clothes before you get into bed Either way, it is usually said casually when enough talking has been done, and one person suggests / announces that they believe it is a good time now, for you both to go to your bed.