A vesting cliff defers the start of vesting. With a standard 4-year vesting schedule and a 1-year cliff, no equity vests for the first 12 months. On the 12-month anniversary, 25% vests instantaneously, and the remaining 75% vests monthly over the next 36 months.
Cliffs protect against equity wasted on bad early hires. The 1-year cliff is the universal default for employee stock options; founders sometimes have cliffs structured differently in the founder vesting schedule.