Edge trigger can be used to trigger on a rising edge (or slope) or a falling edge in the signal. Which edge is being triggered on, can be selected.
To trigger on an edge, the system has two adjustable levels, the arming level and the firing level. The trigger system will constantly compare the input signal with the two levels.
As soon as the input signal passes the arming level in the specified direction, the trigger system is "armed" and ready to detect a passing of the firing level. When triggering on a rising edge, the arming level must be passed in an upward direction, so the signal must first be smaller than the arming level and then become larger than the arming level. When triggering on a falling edge, the arming level must be passed in a downward direction, so the signal must first be larger than the arming level and then become smaller than the arming level.
When the input signal passes the firing level in the specified direction and the the trigger system is armed, the required edge is found, the trigger condition is met and a trigger signal is generated ("fired"). When the input signal passes the firing level but is not armed, nothing happens and the system will continue monitoring the input signals.
In the image below, rising edge triggering is shown. For falling edge triggering, the firing level and arming level will be swapped.