The phrase "hot coolant" refers to liquid in a vehicle's cooling system that has been heated to a high temperature. It serves to cool down the engine and prevent it from overheating. Full definition
The hose heater control valve opens and allows hot coolant from the engine to blow into the heater core. (yourmechanic.com)
First thing the engine does is warm itself, and it provides hot coolant through the heater core while sending charge to the battery. (hybridcars.com)
The heater assembly will have a shut off valve which operates to prevent hot coolant from the engine reaching the heater radiator. (mechanics.stackexchange.com)