means "it's a deal!" or the one who “说了算” means ”his words work" “he is in charge”

“说了算”describe someone be in charge, others need to obey this guy. 这里他说了算 means " he is the leader here"

make the desicion

我说了算means I'm calling the shots.

就是说话要算数，不能言而无信，一言既出，驷马难追。