A Psionic simulation deck is a type of room commonly found on Protoss space vessels and training structures.
The room is mainly used for recreation and research purposes. Through the use of holographic technology, the simulation deck can replicate virtually anything within certain parameters (Star Trek: Holodeck). For research purposes, most scientists and technicians use simulations to test out new technologies and weapons. Due to lack of resources, this has greatly helped nomadic fleets survive on what little they have.
Much like the Terrans use of Zerg-bots, holographic simulations are created to train troops to combat strains that have already been encountered. On higher levels, the simulations are charged with psionic energy becoming more solid. While this increases the chance of injury, many highly skilled Templar believe it also hones combat prowess with precognition skills faster (Marvel: Danger Room). As an added incentive, trainees are encouraged to score high for prizes such as permission to use resources to customize armor, weapons, and fighters (Dead Space: Firing Range).