Suppose you get a stock grant worth X dollars, vesting over Y years. At the time of the grant, you get nothing. A year later, you get X/Y dollars worth of stock. Another year passes, and you get another X/Y dollars worth of stock, and so on for Y years in total. If at any time you leave the company, any stock that hasn't vested disappears into the aether.

Sometimes, you'll hear this described as "golden handcuffs". You get more money, but it is money that disappears if you ever leave the company, providing an incentive to stay. This is, fundamentally, a wrong way to view the stock grants. Instead, they should be seen as a temporary raise of X/Y dollars per year, that expires after Y years. The money isn't yours until the stocks vest, and until that point is nothing more than a commitment to give you stock at a later point.

Yep, vesting. If I tell you I'll give you employment where you get 20% of your annual salary this year, 120% next year, and 70% for the next few years (but ofc subject to market forces), that doesn't compare well at all to someone else's base salary, especially if you quit that first year, which many do.

You get fired or have a mental breakdown and quit before your shares vest.

The shares are worth less in 4 years , etc.