A thin metal rod that confirms the burners actually lit; if it can't sense flame within seconds, it shuts the gas off. A dirty flame sensor is one of the most common 'furnace starts then dies' causes — and one of the cheapest fixes.
Heating terms here point to gas furnace vs heat-pump trade-offs through Maryland cold snaps, AFUE efficiency, heat exchanger safety, and keeping older systems reliable in the region's mixed housing stock.