TRIAC dimmers are the standard 'wall dimmer' used in many households, and connect to the 120V AC.

A 120V TRIAC Dimmer (i.e Leviton 6631) will connect to the 120V, then feed the input of the Driver.

The output of the driver is then connected to the LED, typically 12V or 24V versions.

The advantage of a TRIAC based dimming driver is low cost, since it's the dominant type of dimmer used in the world, and does not require a separate dimmer control circuit as in DALI, DMX or 0-10V, but uses the same 120V AC feed, and superimposes a Phase signal onto the 120V itself, which causes the dimmer to vary to output Voltage, i.e from 12V down to 0V