These lines are significant in this poem because they alert us to the fact that he is blind, first of all, and second of all, that his blindness has not kept him from living and enjoying life. The last line in this passage, "his face takes sunlight like thirst," indicates that he is a man who is thirsty for life; he has an insatiable appetite for living, for the beauties of life in any form that he can get them, and enjoys it thoroughly. He can't see, but what he can do is feel. So, he feels the sunshine on his face, and absorbs its warmth and comfort like he is thirsty for it. That indicates that grandfather is a man who lives life deeply, is grateful for what he has, and hasn't let being blind dampen any aspect of his life. The line "his deck chair settles our rug on the stones" indicates that his grandfather is a comforting presence in his life that grounds him, that keeps him from blowing away or getting lost. His chair kept their rug settled, kept is still, kept is where it should be, and perhaps grandfather was a man that did that in his life. Perhaps grandfather kept him where he was supposed to be, by giving him advice, comfort, and a listening ear.
The word choices symbolize a man who is very happy, grateful for life, able to feel its joys, and who is a stabilizing and settling force in the lives of his grandchildren. I hope that those thoughts help; good luck!