"Fyah caan cool" (often written as Fyah cyaan cool) translates literally in Jamaican Patois to "Fire cannot cool" or "The fire cannot be put out."
In Rastafarian symbolism, "Fyah" (fire) represents spiritual purification, truth, righteousness, and the burning away of corruption or negativity (often referred to as "Babylon"). Saying the fire can't cool means that truth and justice will always prevail and cannot be suppressed.
In reggae and dancehall sessions, shouting "Fyah caan cool!" is an exclamation of intensity, excitement, and approval. It indicates that the vibe, the music, or the performance is hit-after-hit heat that isn't slowing down.