So I’m clearly ignorant on this topic - what exactly IS a pistol brace?
It's a device that fits over the buffer of an AR-style lower receiver, which as a bracket/strap/other mechanism that allows the rear of the buffer to be secured to the shooter's forearm, in order to allow the firearm to be shot single-handed, like a pistol. This allows owners to legally fit a pistol-length (<16") barrel to an AR without running afoul of the NFA as a short-barreled rifle. Or at least it did.
Most of them look like a very short rifle stock, and some idiots have posted YouTube videos of them using braces like rifle shoulder stocks.