A restricted stock unit is a promise, not a share. Your employer says: stay with us, and on these dates we will hand you this many shares. Until a date arrives (called vesting), you own nothing you can sell. Once it arrives, the shares are yours, the same as if you had bought them.
RSUs matter because for many people they are the second-largest piece of pay, and the most misunderstood. A grant of $200,000 vesting over four years is worth about $50,000 a year, paid in stock, at whatever the stock is worth on each vest date. If the price doubles, so does that year's pay. If it halves, so does that.
The tax part surprises people. On the day shares vest, their value counts as ordinary income, like a cash bonus. Your employer usually sells some shares to cover withholding, typically at the 22% flat rate for supplemental wages (2026). If your real tax rate is higher, you owe the difference in April.
A common mistake is treating vested shares as a windfall to be left alone. They are pay. Deciding whether to hold or sell is a real investing decision, not a default. In Allocate, a vest is an expected income event with its own date, so the plan counts it the way it counts a paycheck.