A founder's stock may include acceleration of vesting - in other words, events that allow all of your unvested shares to vest instantly.
There are two varieties of acceleration:
Single triggers add one barrier to acceleration - when the company is sold
Double triggers add two barriers to acceleration - when the company is sold and when the founder is terminated without cause (usually within 1 year after the sale)
Single triggers are more founder-friendly because unvested shares will vest instantly on a sale, but can be an uphill battle with buyers because they will have to figure out another way to motivate and retain founders. To see some other terms that may cause an uphill battle to buyers or investors, read our article on cap table mistakes.
Double triggers are less founder-friendly and more common, because the buyers don't have to worry about founders with unvested shares leaving the company immediately after a sale. In certain cases, if a buyer changes the founder's responsibilities significantly, the founder may leave for "Good Reason" and still gain the benefit of double trigger acceleration.