Technology that converts digital text to audible speech. In other words, it allows a device to "talk" to the user through its speaker.
For example, in a phone, text-to-speech might announce a caller's name when they call instead of playing a ringtone. It might also read a text message aloud, or speak the names of menu items as you scroll through them.
This feature can be useful for hands-free use of a phone while driving, allowing the driver to keep eyes on the road. It is also very useful for the vision-impaired.