(Answer adapted from "What are examples of exit strategy"). One exit strategy in a start-up is an IPO (Initial Public Offering) of the shares. The shares are sold on the public market often at multiples above what was initially invested, thus returning a successful exit to investors.
Another strategy of an exit is mergers and acquisitions by another company. For example, Facebook buying Instagram is an acquisition that benefitted both the founding team and its investors profoundly.