This feature was exclusively part of a joke version and is not currently planned for addition to regular Minecraft. It is documented here on the wiki due to being officially-released content by Mojang Studios, even if not present in a final version.
Reality vision was equipped via the helmet slot. When equipped, it showed a minimap in the bottom-center of the screen (which moved when the player's camera moves). The minimap showed all chunks that were currently rendered. The player's location was represented by a green beacon beam marker on the map.
It displayed a vision overlay (similar to the pumpkin overlay). The overlay would change to a new overlay if the player went into third person (unlike the pumpkin overlay), which made the screen appear like an old television screen. The overlay was called cantseeshitcaptain.png in the assets folder. It activated a shader when equipped (scan_pincushion).