Turning the potentiometer counterclockwise raises the input voltage Uin...
Thereby the voltage at the output switches to (Uout = 9 V). The LED lights up.
A characteristic of the Schmitt trigger circuit is the fact that the emitters of the transistors are coupled. This ensures that T2 remains a little bit longer turned on while T1 already turns on. Only when the input voltage reaches Uin = U1, the T2 turns off suddenly.
The voltage U1 is referred to as the ON threshold.
What happens, when you turn the potentiometer again in the other direction...?