802.1p tagging is a mechanism to mark ports at layer 2 with a Class of Service (CoS) value to help implement QoS at layer 2.
A good example of where you would find 802.1p implemented would be VoIP installations. Implementing 802.1p helps to prioritize voice traffic within a switched environment. Without this, the VoIP packet coming into the switch would be treated no differently than say a packet containing large amounts of data to be transmitted. If the VoIP packet had to sit in a buffer and wait to be transmitted, while the larger packets for the file transfer are sent, you could experience jitter while trying to communicate to the remote station. By marking the VoIP traffic with 802.1p, all switches that support this feature can give the VoIP traffic a higher priority to be transmitted and thus reduce the delay ini getting those VoIP packets to their destination.
Here is a link that may help to better get an idea of how 802.1p works within a switch.
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/2950_wc/scg/scg_vlan.htm#xtocid2248340
Steve
