A minimum period you must work before any equity vests — leave before the cliff and you earn nothing.
The cliff is a waiting period at the start of a vesting schedule during which no equity vests. The most common cliff for founders and employees is one year. If you leave before the one-year mark, you receive zero equity. At the cliff date, the amount that would have vested during that waiting period all vests at once — typically 25% of total equity for a standard four-year/one-year schedule.
The cliff exists to filter out early departures. Without it, someone could join a company, work for two months, and walk away with 4% equity (2/48 of a four-year schedule). With a one-year cliff, they'd walk away with nothing, keeping that equity in the pool for people who stay and contribute.
After the cliff, vesting typically continues on a monthly (or sometimes quarterly) basis until the full vesting period is complete. Important: the shares that vest at the cliff represent all the equity from the start of the vesting period through the cliff date — it's not a bonus, just a catch-up.