Most likely, you will configure priority queuing (PQ) if you want more important, or higher-priority, traffic to always be sent  before less important, or lower-priority, traffic. PQ creates four distinct queues of decreasing priority. The PQ queues guarantee
that higher-priority traffic will always be sent before lower-priority traffic; however, this strict prioritization can cause lower-priority  traffic to experience network starvation.

There are four basic queuing mechanisms available on most Cisco hardware platforms: FIFO, PQ custom queuing (CQ), and weighted fair queuing (WFQ) With FIFO queuing, packets exit a queue in the same order that they entered. Because of its simplistic nature, a FIFO queue is particularly efficient but can introduce considerable amounts to delay and jitter because of the inherent lack of traffic prioritization. Interfaces on Cisco devices generally use FIFO queuing by default.