The fire-message is one form of communication for Shadowhunters, who used to refrain from the usage of electronic gadgets and phones, and is used primarily when they are in Alicante, as most modern devices are still impossible to use within the city wards.
Fire-messages were developed some time in the 21st century. Before then, the Clave used a phone—the only working telephone in Alicante—in the Consul's office in the Gard that was enchanted by Ragnor Fell.
A fire-message is sent by burning a piece of paper with the message. The Nephilim do this by Marking the piece of paper with the rune for fire. A warlock's fire-message is apparently powerful enough to bypass the wards around Alicante. It is possibly better to be sent out from a wide, open space.