What is a bridged interface and how would one be used?

From PFSenseDocs

A bridged interface is one that can filter traffic without pfSense being involved in the IP layer of the connection. This is commonly referred to as a "transparent firewall". For an example bridging configuration, see this in the m0n0wall documentation.

Previously in m0n0wall there was a enable filtering bridge setting. Please note that our filtering bridge is enabled by default.