Ruth's pledge to Naomi: "Donít urge me to leave you or to turn back from you. Where you go I will go, and where you stay I will stay. Your people will be my people and your God my God. Where you die I will die, and there I will be buried. May the Lord deal with me, be it ever so severely, if even death separates you and me."
Boaz has to break it to Ruth that there's a closer male to her dead husband's family, meaning she would have to marry the other man to fulfill the Levirate marriage. But in the meantime, that night he allows her to stay with him and "bundle up" (i.e., sleep together for warmth).
And the next day, Boaz figures out an argument to get that other man to pass on the Levirate obligation - pointing out that any child with Ruth would split the inheritances for that man's other sons - clearing the path for Boaz to marry Ruth after all.