This is a set of advanced visual effects, particle systems, 3D models of monsters, ammo boxes, game items and weapons. Realistic blood splashes, monster gib skins, reflections, dynamic lightning and so on, and so on.
Absolutely incredible work, a must-have for all JDoom users.
Locate the installed Doomsday engine folder (most probably in 'C:\Program Files\Doomsday')
Open 'snowberry\addons\' subfolder.
Unpack the 'jdrp.box' folder from the JDRP archive to the 'addons', so the resulting directory structure will be something like this, depending on the Doomsday folder in your system:
Start Doomsday, select 'Shared Settings'->'Addons' and check the checkbox near 'jDoom Resource Pack'.
Select the game type ('Doom 2' for example) and click 'Play' to start the game.