Public Test Realms, or PTRs, are restricted but publicly-available servers that companies like, say, Blizzard, use to test updates to their online games before rolling them out to the world at large. It's like an open beta clubhouse: In exchange for accepting the risk that things may go sideways from time to time, players can get an advance look at the latest changes coming to their online game of choice. There are PTRs for World of Warcraft, Diablo 3, and Heroes of the Storm, just to name a few chosen entirely at random, and soon, based on data dug up by Hearthpwn, there may be one for Hearthstone.
The site said that an update to Battle.net that was pushed out last night contained encrypted data for a Hearthstone Test Client. That doesn't necessarily mean that a Hearthstone PTR is on the way, and in fact Lead Designer Bed Brode said last month that there would be “real downsides” to setting one up, because new cards would be “fully spoiled” long before being released to the public. But as Hearthpwn points out, a PTR could serve a number of useful functions without spoiling anything, like testing bug fixes and balance tweaks for existing cards.
I've reached out to Blizzard for more information, and I'll update if and when I receive a reply. Given that World of Warcraft, Diablo III, StarCraft II, and Overwatch all have PTRs, it's certainly at least possible that Hearthstone is joining the bunch, if not probable.