See the oscillogram and the shape it makes it the middle. That's the hysteresis. The difference between the turn on threshold level and the turn off level. Imagine it this way. The trigger activates at 1.5V but turns off at 1.3V. The range may vary widely and is adjustable. That difference can be useful in some applications like if you're trying to build a thermostat that turns on at one temperature 20degrees and turns off at a different (25degrees). You just need the schmitt trigger as it is and a NTC thermistor voltage divider set up at the base of the first transistor. The temperature lowers, the resistance of the thermistor increases until it reaches the first threshold. That's when the trigger turns on a relay that engages a heater which in turn heats up the thermistor. The temperature increases, the resistance decreases until it brings it to the second threshold that disengages the heater. And this is kept in an endless loop or until you turn the device off. Let me make a little line of sequences here if you didn't understand me.
20degrees -> trigger reaches first threshold and engages the heater -> heat goes up -> 25degrees -> trigger reaches second threshold and disengages the heater -> temperature goes down to 20degrees again and the whole things start over again.
