This refers to the video compression standard used. It’s a very common format that works on almost any modern device.
The string you provided is a "release name" that tells a downloader exactly what they are getting: Season 3, Episode 1.
This specific episode is a major turning point for the series. It follows the team as they relocate to after a tragic loss. It introduces iconic characters and sets up the Red Hood arc, which is a fan-favorite storyline from the DC comics.