This module advertises STUN/TURN service to clients via XEP-0215 (it utilizes mod_external_services to do so, which will be automatically loaded by this module and must be available).
This module is included with Prosody 0.12 but made available here for older versions.
Documentation is available on the Prosody site.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_turn_externalFor earlier versions see the documentation for installing 3rd party modules