A campfire was a fire lit in the outdoors while camping. This fire would be used to cook food or keep warm.
Some inhabitants of Bozeman, Montana used campfires to cook their meals in 2063. (Star Trek: First Contact)
In 2287, James Kirk, Leonard McCoy, and Spock lit a campfire in Yosemite National Park. They used it to cook Southern baked beans, toast marshmallows, and to sleep near. They also sang "Row, Row, Row Your Boat" around the fire. After their mission to Nimbus III and the Great Barrier, they did so again. (Star Trek V: The Final Frontier)
The musket-wielding aliens created by Q in 2364 had a campfire near their tents. (TNG: "Hide and Q")
Data once described campfires as "thermal radiation and carbon dioxide emissions, indicative of combustion". He did this in 2366 while in search of the Acamarian Gatherers on Gamma Hromi II, who maintained several campfires. (TNG: "The Vengeance Factor")