Part of a video streaming service. Needs to be enabled to watch SpeedRunsLive and various other sites. Streamed content is user dependent and therefore may contain inappropriate content.
At my guess it stands for
JustinTV Network.
This is the side which seems to be owned by justin.tv to host its media and streams for the main website. Other sites also might use to for the same reason.
Good (background) site. Needs to be trusted for justin.tv to work with script blocking addon/programs.