"Coolant circulation" refers to the movement or flow of a liquid, called coolant, in a system or machine to help regulate its temperature and prevent overheating. Full definition
The activation condition and continued coolant circulation period are constantly calculated from the time of starting the engine. (mechanics.stackexchange.com)
Lack of coolant circulation is usually caused by 3 things: closed thermostat faulty water pump blocked passage or hose Since you changed the thermostat, we could likely rule that out. (mechanics.stackexchange.com)
To provide back - up for the mechanically driven coolant pump at low engine speeds and to ensure adequate coolant circulation. (mechanics.stackexchange.com)