How Does the Fastpass+ Work?

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Last time I went to WDW was in 2012 and all I did at that time was insert my room key at the fastpass kiosks and was given a time ticket. Now I got my Fastpass+ wristbands in the mail and honestly I have no idea how they work. I've seen that I can reserve a time for a ride up to 30 days in advance? I'm completely lost. Can anyone give me the run down of how they work and what I need to do? Thanks!