The whole point of Sonnet 2 is to talk the young man it's addressing into having a kid. Shakespeare starts out by trying to scare this young man a little bit, to make him think about what it will be like to be old. He warns him that even though he is handsome now, his good looks just won't last. He'll get wrinkles, his eyes will sink into his head, and his blood will turn cold.
After this scary speech, he tells him that there's a way out, a chance to cheat old age and death. How, you ask? Well, by having a son and leaving a copy of himself.