Q. I do not understand what "propagation delay" is.
A. When the inputs to a gate or flip-flop change, it takes a certain amount of time, usually measured in nanoseconds, for the output to respond to the change.
This called the gate of flip-flop's propagation delay because the input signal is delayed in going through the gate. Most of the propagation delay is caused by the internal capacitance of the transistors inside the IC.
It takes a little time from them to charge or discharge in response to the changing voltage on the input of the device. The propagation delays of the gates and/or flip-flops in a digital circuit determine its maximum operating speed.