Negative Edge Triggering.
It is a type of Triggering circuit, which will get activated at the time of negative going input signal[Trigger signal].
ie; When the Trigger signal Falls from High (logic 1) to Low (Logic 0).
Positive Edge Triggering.
It is another type of Triggering Circuit, which will get activated at the time of Positive going Input signal [Increasing].
ie; When the trigger signal rises from Low (logic 0) to High (Logic 1)
In this type of circuits, the circuit may get activated Either at the High or Low Level.