It basically means that I will love you forever. 世界末日means the end of the world. It is used to refer to some time far in the future. However in the context, it says 不够远， which means not far enough. Here he is saying, even it's the end of the world, i will not stop loving you, that the day the world ends will not not be the end of my love for you. To make it easy, you can read it as "MY LOVE FOR YOU WILL OUTLAST THE WORLD." If you want to make it more simple, it just simply means "MY LOVE FOR YOU IS TIMELESS" Read the original sentence as "世界末日不够远,(put a comma here ）不是爱你的终点" might help you understand better. I know it's a little bit confusing, hope it helped you.