The reactor is used to generate power for the ship. Shutting down the reactor helps to mitigate heat from the reactor but shuts down all power output. Overloading the system can help create additional power when necessary to run multiple systems in a crisis situation but will more quickly heat the reactor. Coolant can be used to cool the reactor. It will be important to balance power output with heat levels necessary to produce the amount of power your ship will need at a given time.