Liquid nitrogen, or N2 in scientific terms, is nitrogen gas that has been cooled to 63.15k or below, at which point the gas will condense into a liquid.
Liquid nitrogen is a specialty of Spacesim. Liquid nitrogen is used for the EEP as a demonstration of how cold it can become on the outer planets, namely Neptune. It has also been used for publicity, usualy involving freezing marshmallows and impressing potential sim members with "Mad Science Skillz!". It is particularly effective as part of the Week O'Hype, mostly to "cook" Liquid Nitrogen Marshmallows, which are very popular.