Before switched ports were commonplace, bridging, which takes place at layer 2 (the Data Link layer) of the seven layer OSI model, was an effective way of connecting multiple network segments into a single broadcast domain, while limiting the amount of traffic that spans more than one segment. For example, consider a network with two network segments; Nodes A and B are on the first segment, and node C is on the second segment.

If we connect the two segments with a repeater, then a packet from A whose destination is B will flood both sides of the repeater, even though B is on the same side of the repeater as A. But consider the case in which we place a bridge between the two segments. At first, the bridge does not know which nodes are on which side of the bridge. Gradually, however, a bridge will learn which side of the bridge a node is on, and packets from A to B will be confined to a single segment. This potentially reduces the amount of traffic on a network.

You can bridge two interfaces in pfSense and form a single broadcast domain. In such cases, two nodes on bridged interfaces act as if they are on the same switch. The main difference is that firewall rules for the bridged interfaces must allow traffic between the interfaces, or traffic will be blocked.