Vesting means that instead of getting all the stock up front, you will earn it over time. Often founding members of start-ups leave the team at an early stage. For companies without vesting, some founders give back the stock voluntarily and some don’t. Vesting is a good protection: you want to be clear from the start about what someone will walk away with if they leave. Many start-ups need to acquire external funding, and, for investors, it is often a refusal criterion if no vesting rules are implemented.