Restricted Stock Units (RSUs) are a company promise to deliver common stock (or cash equivalent) to the employee on a future vesting or liquidity event. They are distinct from stock options (rights-to-buy at a strike price) and restricted stock (actual share ownership from grant), used heavily at late-stage private companies (often via double-trigger structure) and post-IPO public companies. It is the modern equity-compensation mechanic at companies past the seed stage where option grants would create immediate tax exposure for employees.
The mechanic of an RSU grant: