Somewhat like making a dinner reservation at a restaurant, FastPass+ allows you to make a reservation to ride an attraction at a Disney theme park. You can request a specific time, such as 7:30 PM, or you can let the FastPass+ system suggest some times.
Each FastPass+ reservation lasts for an hour. Thus, if you make a FastPass+ reservation to ride Space Mountain at 7:30 PM, you have until 8:30 PM to use it, or change it to something else. Just like a restaurant reservation, your FastPass+ may be canceled if you don't show up on time. (In practice, you can usually be up to 5 minutes early or 15 minutes late to use your FastPass+ reservation.
Buy your tickets before you arrive and you can choose your FastPass+ experiences up to 30 days in advance of each day of your theme park visit. In order to start making advance FastPass+ selections, you’ll need to link your park admission to your account and add the people you’re planning with to your Family & Friends list. When you’re ready, click here to link your tickets
You may see advertising for ticket sellers offering deep discount, or even free tickets. Such deals are possible but always involve hours listening to a timeshare presentation and a hard sell before you can get them. Remember if a deal looks too good to be true, be sure to find out up front what the catch is so you know what you're getting into.
Also be aware of unscrupulous ticket sellers selling used tickets with remaining days. You will probably not be able to use these when you get to the park and just end up loosing your money. It's always safe to buy tickets directly from the parks. There are also trusted authorized ticket sellers you can use. There is an advantage of buying your tickets before you arrive as you will not have to wait on any lines to buy them. You will be ready to enjoy the parks as soon as you arrive. Also you will be able to use the FastPass+ features described below.