It means that your normal vest schedule doesn’t start for one year, so if your initial stock grant vests quarterly, none vests at all for the first year and then 25% vests all at once after year one, then the other 75% of it vests quarterly over the following three years. Essentially it’s a small, localized version of golden handcuffs for the rank-and-fils, since it means if you leave the company within the first year you get no stock.