Fire Geysers are monster spawners you will encounter in the Swamps of Valheim. They spawn Surtlings and can be used to build Surtling Farms.
Fire geysers seem to spawn much more frequently in larger swamps and are rarely seen in small swamps. You can use this as an indicator of how large a swamp is before fully exploring it.
Stepping too close to a fire geyser will catch you on fire and apply the Burning condition. However, it will also keep you warm, which can be nice since the swamp gets cold and wet at night.