Answer:
An open loop system has no feedback to control its operation. For example, if you have a furnace that turns on when a certain low temperature is reached. The furnace heats the house, but there is no sensor to determine when the desired temperature is reached. It just keeps running until it is turned off manually. In a closed loop system the system would sense the desired temperature and automatically turn off.
There is also a principle called hysteresis. In the furnace example, you don't want the furnace to turn on immediately after it stops because of a small temperature drop. This delayed turn on feature describes the hysteresis in the system.