Text to speech is a service that converts written text into audio, which can be played back through a computer's speakers or heard over a phone. All texts sent to the service are converted into an audio file that can then be accessed and listened to by a user from any device with an internet connection. This service has been around since the 1990s but has recently started to become more popular due to its versatility.
In other words, text-to-speech is a computer program that turns written text into speech. Text-to-speech is sometimes called voice synthesis, or speech synthesis.