Why vesting matters to a first-time founder
Without vesting, a co-founder who leaves after a few months can keep a large stake they never earned, and you are left building a company partly owned by someone who no longer contributes. That dead equity worries investors and can stall a funding round until it is fixed. Vesting ties ownership to the time and work people actually put in, which protects everyone who stays.
Examples of vesting
You and your co-founder are building the first version of your app, and you agree they will hold 40%, with vesting over four years and a one-year cliff. You write the dates down before the first screen is finished, so the first year earns nothing. Six months later they leave, and none of the 40% has vested. If they had stayed two years, about half of it would have. What happens to the rest depends on whether you used options or shares that were already issued.
Common misunderstanding
People assume an agreed split means the shares are already theirs to keep if they walk away. Unvested options usually lapse when someone leaves. Already-issued founder shares may instead be subject to transfer or buyback rights under a reverse-vesting agreement. The documents and applicable law determine the outcome. Dilution is a separate question.
When to use it
Agree a vesting schedule when more than one person holds equity, including co-founders, and do it before an investor asks why there is no schedule. If the shares were handed over with no timetable, put one in place before the relationship is under strain.