There should be a sign on Hannibal Lecter's cage: do not feed the cannibal.
When Lecter is taken from his cage, he's forced into a mask to keep him from getting too…snappy. The mask isn't a symbol for anything. It doesn't stand in for oppression or Lecter's imprisonment (the straightjacket restraining him does a good enough job of that on its own); it just looks creepy, especially in the film's numerous ghastly close-ups.
Paired with Anthony Hopkins's scary unblinking eyes, this mask worms its way into your nightmares. Lecter is intimidating and dangerous even when he's completely helpless, and that's terrifying.