Someone has only glimpsed or experienced the beginning of something that will become even more exciting, impressive, or important. (The word "ain't," a colloquial form of "hasn't" or "haven't," is often used instead.) I could tell your parents are already impressed with the house, but they haven't seen nothing yet! A: "Wow, there is so much going on in this city all at once!" B: "Just wait, darling, you haven't seen nothing yet—it only really comes alive at night!"