In most data networks, traffic arrives in fluctuating bursts from multiple links, which are all destined for the same output link (itself only having a finite capacity), leaving a router with more packets than it can immediately deliver.
For example, if you have two input 45Mbit T3 links which are bursting to near 100% capacity and the traffic from these links must converge to a slower 1.5Mbit T1 link the router will need to internally buffer or drop some of the packet data until they can be sent onwards. Under these circumstances packets attempting to pass through a router experience additional delays. Such a router is said to be suffering from, "transient congestion."