source of fire; place where fire could start
Refers to a potential fire hazard or the origin of a fire. Often used in safety contexts, such as checking 火の元 before leaving home or after a disaster.
Please check for fire hazards before going to bed.
After an earthquake, you need to be careful about potential fire sources.
Compound of 火 (hi, 'fire') + の (genitive particle) + 元 (moto, 'origin, source'). Literally 'the source of fire'.