Proving Ground is a scenario where you can test your ability to tank/dps/heal. If I remember correctly, it used to be required to finish to do certain group finder content (I vaguely remember needing to do it when the MC anniversary event happened, maybe I'm making that up) but at the moment it basically means nothing.

I checked it out recently-ish after seeing someone's "Proving Ground: Gold" or whatever it said when I was making a Mythic group, but it's extremely undertuned I think (or maybe I was overgeared) to the point where it really doesn't matter. It certainly doesn't matter for Mythic or Mythic+.

You can access it at your garrison, though. Even at garrison level 1. The person by your mission table has a dialogue option to queue for it.