A digital trigger is usually a TTL signal with two discrete levels: a high and a low level. When the signal moves from high to low or from low to high, a digital edge is created. There are two types of edges: rising and falling. You can produce Start or Reference Triggers from the rising or falling edge of your digital signal.
In the following figure, the acquisition begins after the falling edge of the digital trigger signal. Usually, digital trigger signals are connected to PFI pins on your measurement device.