If you leave after 12 months and one day, you keep a quarter of your initial stock grant.

Assume the stock grant was 48,000 if you have after the day after you completed 12 months, you get to keep 12,000 shares not 24,000.

In terms of taxes, you have to file an “83b election” with the IRS to prevent being liable for “paper wealth” that’s not really cash

Technically you are granted all 48,000 shares at a nominal value like .00001 and the company has the automatic right to buy back the shares you do not vest at that same nominal value