FASTPASSES are offered only on certain popular rides where there is often a long wait, and they can accommodate a separate FASTPASS entrance.
At these rides, a sign will tell you how long the standby wait is and what time the FASTPASS return is currently scheduled, so you can see if it will fit your schedule. Follow the FASTPASS Distribution sign to find the FASTPASS machines where you insert your park ticket and receive a time-stamped FASTPASS ticket with your appointment to return later in the day. You can only get one FASTPASS per park ticket, but to save time, one person can take the whole family's tickets to get FASTPASSES while the others are waiting in line at another ride.
You cannot pick up another FASTPASS on a different ride until your first appointment has passed or after a designated amount of time printed on the ticket. For example, your FASTPASS might say that your appointment to return to this ride is in 3 hours, but you can pick up another FASTPASS in 2 hours. The times are calculated automatically based on the projected ride load and distribution of passes so far for that ride.
TIP: First thing in the morning, the FASTPASS appointment may be in just 15 minutes, allowing you to get another FASTPASS then and use them both later in the day when lines are longer so you can ride other non-FASTPASS rides early. The FASTPASS gives an hour window printed on it, but you can generally use the FASTPASS any time after its start time (I confirmed this with cast members at the park).
Using the FASTPASS system, especially in conjunction with Ridemax, can save hours of time waiting in long lines, allowing you to fit much more into your day.