I suspect (and somebody else please correct me if I'm wrong) that it refers to how much of your options vest over a period of time.

So you'd get 10 percent of your options after one year, another 20 percent at the end of your second year, another 30 percent at the end of your third, and the remaining 40 percent at the end of your fourth year.

Note that in order to collect even half of your options, you need to work there for three years.