Hi, roserae! Fastpass can be kind of confusing at first, but it is a really great system, so you'll definitely want to take advantage of it.
Here's how it works:
Some attractions throughout the parks are designated fastpass attractions. This means you have the option of either waiting in line or getting a fastpass and coming back during your designated time window (about an hour) for a usually minimal wait. You get a fastpass by inserting your park ticket in the attraction's fastpass machine, which is usually located near the entrance of the ride (if you can't find it, ask a CM). You may only hold one a fastpass ticket for one attraction at a time.
Usually come back times are an hour or more after you get your fastpass. Times are posted above the machines and printed on the fastpasses, so you'll always know when you should be back. If an attraction has a short return time, that means the line is probably also short, so you should go ahead and ride.
Fastpasses for the more popular attractions sometimes run out, so you should get one early if you plan to take advantage of fastpass. If you end up getting a fastpass that isn't valid for several hours, you will be able to get another pass for a different attraction. For example, if you get a fastpass for Splash Mountain at 4 p.m., but your window doesn't start till 8:30 p.m., you'll be able to get a fastpass for another attraction around 6:00 p.m.
I know it sounds kind of confusing, but it will all make sense when you get there. Good luck!