The phrase "will be named" means those born through Isaac are the ones God considers to be the descendants that he promised Abraham. This can be stated in active form. AT: "Isaac is the one who will be the forefather of the descendants I promised to give you" (See: [[rc://en/ta/man/translate/figs-activepassive]])
The word "nation" means God will give him many descendants so that they become a great nation of people. AT: "I will make the servant woman's son also become the father of a great nation" (See: [[rc://en/ta/man/translate/figs-metonymy]])