This property is used to fade effects in and out over time. The values are in terms of "time/alpha", each separated by a "|". The TimeToLive property says how long the object will actually live. This argument is a Percentage Series type of argument. The alpha is how opaque the effect object is, a value from 1 (fully opaque) to 0 (fully transparent). The alpha is then linearly interpolated between the alphas set for the given times.
Source and more details