A Bridge joins two or more interfaces to the same layer 2 (broadcast/collision domain). See What is a bridged interface and how would one be used? for more information on their uses and capabilities.
Bridges between interfaces are listed and managed at Interfaces > (assign) on the Bridges tab.
When editing a bridge, Ctrl-Click to select the Member interfaces from the list of available interfaces and type a Description.
Advanced options may be used to enable features like Spanning Tree or other less common features of bridges.
Once a bridge has been created, it can be assigned under Interfaces > (assign) as any other interface.
By default, traffic is filtered on the member interfaces and not on the bridge interface itself. This behavior may be changed by toggling the values of net.link.bridge.pfil_member and net.link.bridge.pfil_bridge under System > Advanced on the System Tunables tab. With them set at 0 and 1, respectively, then filtering would be performed on the bridge only.
NOTE: Only one interface on a bridge should have an IP address! Do not add multiple IP addresses in the same subnet on different bridge member interfaces. Other interfaces on the bridge should remain with an IP type of None.