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.
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.