Steganography hides the existence of a message, where encryption only hides its contents. Data is concealed inside a carrier file — most commonly by subtly altering the pixels of an image or the samples of an audio file — so the result looks completely ordinary to anyone who isn't looking for it. Two people can exchange a normal-seeming photo that secretly carries a hidden payload.
For investigators, steganography is both a threat and a clue. It is used to smuggle data past filters and to conceal communications, so unusual file sizes, patterns, or artefacts can hint that a file carries more than it appears to. Detecting or ruling it out is part of thorough file analysis, alongside reading a file's metadata.