Fastpasses for rides: you use your park ticket at the fastpass machine for the ride you want. It will give you a little paper with a return time window to come back & ride and will also tell you at what time your park ticket is eligible for another FP. The basic premise here is that you are waiting in a virtual queue for the ride while you can go do something else. When FPs run out for the day on a ride, they are gone. FPs commonly run out on RSR earlier in the day than other FPs which you can usually acquire even into evenings (depending on crowd levels & ride popularity).
Fastpasses for shows: these are not linked to the ride FP system, you can get a show FP & then immediately acquire a ride FP.
Anna & Elsa meet & greet FP: I can't help here, but I'm sure others on the DIS can.