Definitions of let out:

• verb:   make (clothes) larger

  Example: "Let out that dress--I gained a lot of weight"

• verb:   make known to the public information that was previously known only to a few people or that was meant to be kept a secret
• verb:   bring out of a specific state
• verb:   express audibly; utter sounds (not necessarily words)

  Example: "She let out a big heavy sigh"