A yellow fireplug
A fire hydrant or fireplug was a device that, when connected with a hose, provided a steady stream of water to enable a fire to be extinguished.
In 2268, a landing party from the USS Enterprise beamed down next to a yellow fireplug at an intersection of two streets in the Northside Territory of Sigma Iotia II. (TOS: "A Piece of the Action")
A yellow and red hydrant was also part of the Dixon Hill holoprogram. (TNG: "The Big Goodbye")