A vesting cliff is a wall at the start of a vesting schedule. For a set period, most often twelve months, nothing vests. On the day the cliff ends, a big block vests at once, typically 25% of the whole grant, and after that the remainder arrives in smaller regular pieces.
Employers use cliffs to avoid handing stock to people who leave in the first few months. For you, the cliff creates a date that matters a great deal. Someone who starts on March 1 with a $100,000 grant and a one-year cliff has $0 of it on February 27 and $25,000 of it on March 1 of the next year.
Two things to plan around. First, the cliff vest is taxable income on that day, and the 22% supplemental withholding (2026) may not cover what you actually owe, so a large cliff can create a tax bill the following spring. Second, if you are thinking about leaving, the cliff date is the single most expensive date to miss.
A common mistake is counting unvested cliff stock as savings. Until the cliff passes it is not money; it is a reason to stay. In Allocate, add the cliff as an expected income event on its date, and the plan will show what it does to the month it lands in.