Infravision is an intrinsic property possessed by dwarves, elves, gnomes, and orcs. Many monsters also have this ability. Humans do not have infravision: to make use of it, they must polymorph into some other monster that has infravision.
Infravision allows a character to detect warm-blooded or fire-based monsters in their line of sight, even if that location is unlit. Such monsters are known as infravisible. Infravision does not detect invisible creatures.