Vesting module provides a means of scheduled balance lock on an account. It uses the graded vesting way, which unlocks a specific amount of balance every period of time, until all balance unlocked.
The schedule of a vesting is described by data structure VestingSchedule: from the block number of start, for every period amount of blocks, per_period amount of balance would unlocked, until number of periods period_count reached. Note in vesting schedules, time is measured by block number. All VestingSchedules under an account could be queried in chain state.
The implementation uses locks which allow tokens to be locked by other pallets that's also using locks, for example, the conviction-voting pallet.