A text-to-speech (TTS) app is a computer program or mobile application that enables users to convert written text into spoken words. These apps are especially useful for people who have visual impairments, reading difficulties, or language barriers. TTS technology has come a long way in recent years, and today's TTS apps are able to produce high-quality audio output that is almost indistinguishable from a human voice.

The basic concept of a TTS app is simple: the user inputs text, and the app converts the text into an audio file that is played back through the device's speakers. However, the process of converting text into speech involves a number of complex algorithms and linguistic rules.

First, the app analyzes the input text to determine the appropriate pronunciation and intonation for each word. This involves parsing the text into individual words and phrases, and then applying linguistic rules and algorithms to determine the correct stress, tone, and rhythm for each utterance.

Next, the app selects a voice for the output audio. Most TTS apps come with a variety of voices to choose from, including male and female voices, different accents, and even celebrity impersonations. Some TTS apps even allow users to create their own custom voices by recording their own speech samples.