Welcome to the Disboards!
A PS stands for Priority Seating. Its sort of a reservation but not exactly.
Not exactly b/c there is no table set aside for you awaiting your arrival. It basically means that once you check in with the hostess, you should be seated in no more than 15 minutes. While if you just walk in without a PS you might have to wait over an hour. Which if you have small children is the last you you want to do.
You can make most PS 90 days in advance. For Character meals you should jump on them as early as you can, for the rest depending on time if year you can do the night before. If you have a very time sensitive schedule I would recommend calling in the PS in advance, plus its always fun to call Disney. gets that vacation feeling pumping!
Have a great trip!
