What does TTY mean in settings

Try searching google, you'll find your answer pretty quickly:)

TTY (TeleType), also known as Text Telephone Device or Telecommunication Devicefor the Deaf (TDD), is a special device required at both ends of the conversation that enables people who are deaf, hard of hearing, or speech-impaired to use the telephone to communicate. TTY works by allowing people to type messages back and forth to one another instead of talking and listening.

Off, unless you are hearing impaired or someone you are calling is and need to communicate over the phone to someone with a TDD device.