The voice tab allows you to select your bot's language and voice.
You can from several speech providers.
You can use the HTML5 voice on web browsers that support the HTML Speech API such as Chrome.
The HTML5 voice will use Android or iOS Speech on mobile apps.
This list of HTML5 voices depend on the platform and OS, and the devices configuration.
A Bot Libre voice is consistent across all platforms.
If you use an HTML5 voice, and the browser or platform does not support TTS, then the Bot Libre voice will be used as a fall back. If an HTML5 voice is set, and it is available it will be used, otherwise any voice for the language will be used, otherwise the default HTML5 voice.