Generally speaking a subscription to these kinds of performances consists of different selections of ballets/operas/musical selections (in the symphony sort) that are placed in subsets such as Tuesday nights, which are less expensive than Fridays, etc. There are also other subsets with titles such as "All Robbins Night" (as in Jerome) or "4 Classics by Petipa" ..... In other words, the subscriber is given a number of choices from which to choose, whether they are based on day of the week, or subject matter. Sometimes subscriptions are geared towards attracting new subscribers or families with children...so the choices within these subscriptions will be tailored to what the company's marketing department feel is appropriate. Some subscriptions may be 2, 3, 4 or more performances throughout the season.
Why subscribe? You get the best choice in seating, there is usually a slightly cheaper price per ticket, and if you keep re-subscribing you can even keep the seats you had, if you like them, for the next season! The other "perk" is that if you are a subscriber, you can exchange your tickets on a certain day for another day if needs be...Which is something I have to do myself for this spring!
Each company will have a brochure that will outline all the details and explain the benefits of subscribing. You need to read them carefully - for example NYCB has a "Fourth Ring Society" and the ticket prices are very, very low...
The other plus about a subscription is that you KNOW you're going to get in the door!
So there's my long winded, somewhat convoluted, answer - hope it wasn't too confusing!