Priority queuing is a platform-independent congestion management method that, as its name implies, provides some packets absolute priority over other packets. The simplest analogy is a grocery store with two checkout lines being serviced by one clerk.
Customers holding a special customer club card can enter one line, and the remaining customers can enter the second line. The clerk cannot check out any customers in the second line until he serves all the preferred customers (the ones holding the club cards in the first line).
This is exactly how routers implement priority queuing. Packets are classified based on a configurable policy and are placed in a queue. The IOS implementation has a total of four queues: high, medium, ...