Hysteresis is the expected deviation or accepted margin of temperature variation from the target temperature.

This set deviation allows for certain amount of temperature fluctuation above and below the target temperature before the necessary heating or cooling is triggered and activated.   As the wort reaches the set temperature allowed above/below the target temperature, the necessary heating/cooling is triggered to bring the wort closer to the target temperature. 

Over time, the temperature of the wort deviates less and less from the target temperature as the wort becomes more uniform in temperature.

Ultimately, while this method for reaching a target temperature may take a bit longer, it does allow for a more uniform cooling/heating of the wort.