Elle a posé toutes ses lettres au tour d'après

In the context of a game of scrabble ‘she (grandma) placed all her letters (on the board) at the (her) next turn ‘.

A long time since I have played scrabble but it appears that is immediately a winning move if the word is correct, as it evidently was here ! 

(Poser carries the meaning of careful, tidy placement rather than just haphazard, unthinking)