Coolant is a liquid or gas that helps to keep machines or engines from getting too hot. It absorbs the excess heat and helps to regulate the temperature, keeping things running smoothly and preventing damage. Full definition
The engine coolant temperature sensor measures whether the engine is warmed up or cool. (en.wikipedia.org)
Signs of coolant temperature sensor failures include sluggish acceleration, hard starting, and a Check Engine or Service Engine Soon Light. (yourmechanic.com)