Propagation delay is the distance of a network wire divided by the propagation speed (or the speed of light, according to some) in computer networking. It measures how long a signal takes to travel from a router to a recipient.
Networks have multiple types of delays. Transmission delays occur when data is waiting to be placed onto the wire. Data, separated into packets, is transmitted through a router, but only so many packets can stay on the wire at once. A transmission delay affects how quickly a packet can enter the network. Although these delays are typically quite miniscule, multiple delays can add up and slow network speed.
A queueing delay, on the other hand, is the wait time to process a job or packet, since a router can only process a certain number of packets at one time. After the packet of data is transmitted onto the wire and processed, it travels the wire and reaches its destination. If that travel is hindered or slowed, a propagation delay has occurred.