A vesting cliff is the period of time before you acquire ownership rights to the equity portion of your total compensation.
A vesting cliff is the period where time is accruing, but you are not yet being granted any of the ownership of equity.
No, vesting cliffs are pre-defined before the contract is signed.
Yes, almost all employees who are granted to receive equity have a vesting cliff baked into the contract. This is to align the incentives of the employee with the company, to try and retain the employee for a key amount of time.
The most common term for a vesting cliff is one year.