WWE has yet to reveal exactly what a Boneyard match is but AJ Styles gave a couple of details on the match during his live Mixer stream.
Styles said that the Boneyard Match will not take place in the ring and there are no rules so anything goes. He also said that there would be a grave involved. Again, nothing is confirmed by WWE but this sounds like an old school Graveyard or Buried Alive Match under a different name.
The Undertaker has wrestled several matches where the goal is to bury your opponents. Some notable Graveyard/Buried Alive Matches include his match the Mankind in 1996, Stone Cold Steve Austin in 1998 and Kane in 2010. There was at least one Graveyard Match in 2000 between Vampiro and The Demon.
As previously noted, the plan is for most of the matches to happen at the Performance Center with a few matches happening at other locations. So this confirms that Taker/Styles is one of those matches that took place outside of the ring so don’t expect spoilers to leak out before the show.