How to rate what I've watched?
Once you've watched an episode, you're invited to rate it (you can turn this off in your account options, with the "Rate what I just watched" option). By giving it a rating, you help fill in the episode's page, and you can also easily find episodes you might want to rewatch (the best-rated ones, or the worst-rated ones if you're a masochist) from the Ratings page of your profile.
By going to a show's, an episode's or a movie's page, you can also give it a rating by clicking the stars below the title. The stars can only be clicked if the show or movie is in your account, or if the episode is marked as watched.

Note: the same system applies from the BetaSeries apps.
These ratings are carried over and displayed on the information page of each episode, show, or movie, where you can also leave a comment.