XDR is a Sram freehub body "standard". It derives from the mountain bike XD freehub body and the "R" is for "Road". XDR was introduced by Sram in 2019 with the introduction of their 12 speed Axs wireless road groupset.
XDR cassettes are mainly found on Sram 12 speed road and gravel groupsets.
XD vs XDR Freehub Body
An XDR freehub body is identical to XD other than having and additional 1.8mm on the splines. This means that an XD cassette will fit onto an XDR freehub body when a 1.8mm spacer is fitted behind the cassette.
Please note the opposite is not true: There is no way of fitting an XDR cassette onto an XD freehub body.