They said they'll be posting in the next few weeks with more details. Honestly it's a form of gear, but with more bells and whistles. For example, some relics will unlock new abilities.
If I had to guess, I assume it will be the thing you progress to after gear is finished. Almost like a new shard shop, but for gear. Our old gear along with whatever new thing this brings will be used to bolster relics. Again, just my guess, but I think they're done trying to balance around gear and want to free themselves up to do something else.