MaxPass is an extra $10 per day, per ticket. There are maybe 15 rides it can be used on (all Fast Pass rides: Splash, Haunted Mansion when it's doing FastPass, Space, Big Thunder, Buzz, Star Tours, it's a small world, Roger Rabbit [I think this still has FP], Grizzly, Incredicoaster, Soarin', Guardians, and Goofy's Sky School [and whatever other FP rides I missed]). You need it for each person in your party that wants to use it to go on any of the above rides. Of course, if you don't want to pay, there's always the Legacy FastPass system (that is, you go to the FP machine, scan your card, get your time and reminder). You can get a new MP every 90 minutes, *or* when you have used your last one, whatever is sooner. You will need a smartphone to use it (since it runs through the Disneyland app), and it helps significantly to have a data plan, otherwise you are stuck looking for a wifi hotspot to book your next MP. You need to be in the parks to activate the MP, but then can book outside the parks--for instance, if you are taking an afternoon dip in the pool at the hotel, you can book a MP for when you will be back in the park (same day only).
Personally, I love it and won't go without it!