Restricted Stock Units are like pretend money that a grown-up gives to a worker as a reward for doing a good job. But unlike real money, the worker can't have it right away. Instead, they have to wait a certain amount of time or meet certain goals before they can get the pretend money.
When the worker gets the pretend money, they can trade it for real money or keep it as stocks in the company. Stocks are tiny pieces of a company that grow or shrink in value depending on how well the company is doing.
So, Restricted Stock Units are kind of like a promise that if you work hard and help the company get better, you will earn some pretend money that could turn into real money later.