Think of the threshold as a toll booth. When the signal passing through a circuit reaches it, the threshold decides whether or not it can continue. Use it to make any sensor Bit (like a temperature sensor) into a trigger Bit!
The threshold is a minimum parts count design, using only a single opamp to compare the input signal to an onboard reference that is set via the potentiometer control. U1 is configured as a non-inverting comparator with approximately 100mV of hysteresis provided by R1 and R3. VR1 provides the comparator's reference. C1 is a bypass capacitor for U1 that sources current to the opamp during fast switching events.