You’re looking at a flexible brass gas supply pipe going into the side of an older heating furnace.
A flexible pipe is very thin wall. The photo shows the flexible brass gas pipe resting on the sharp edge of the sheetmetal opening. When the furnace is turned on, warm air is pushed into the house by a furnace blower motor which sets up mechanical vibrations.
These vibrations can result in the sheetmetal edge cutting through the flexible gas line during operation… possibly leading to a gas explosion and fire.