A boom is essentially a horizontal pole used to position a microphone or other accessory closer to the subject but just out of shot. It’s used widely when shooting movies or videos.
But some tripods also offer a ‘boom’ mechanism for extending the camera horizontally from the tripod to photograph objects from above or those where it’s not possible to get the tripod close enough (because the tripod legs are getting in the way).
Some tripods can take special horizontal booms where the tripod head (and the camera) is mounted on the end of the boom. Others have a design that allows the centre column to be rotated into a horizontal position.