This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
The coolant was used for the subspace warp field propulsion system on mid-24th century Federation starships, including the USS Enterprise-D. It prevented the engines from overheating by removing excess heat. Engineers were taught safety measures in technical manuals such as Teratogenic Coolant Safety Protocol when working around and with this gas. (TNG: "Booby Trap", okudagram)