A bridge has two primary functions: filtering and forwarding frames.
Filtering is the process of discarding a frame when a bridge detects that the frame does not need to travel any further. As the visual depicts, a frame traveling from the LAN adapter in Station C to the LAN adapter in Station B is seen by all LAN adapters attached to that LAN, including the LAN adapters in Station B and Port 2 of the bridge. When the bridge sees the frame and notes that it is intended for a station reached via Port 2, it has enough intelligence to know that Station B has already seen the frame. There is no need for the bridge to pass the frame to the next LAN, so the frame is filtered (i.e., not allowed to pass).
Forwarding is the process of copying a frame from one LAN to another. In the lower example on the visual, a frame traveling between the LAN adapter in Station C and the LAN adapter in Station A is seen by all the LAN adapters attached to the LAN on the right, including the one in Station B and Port 2 of the bridge. The LAN adapter in Station B ignores the frame because the destination address in the frame is not the local MAC address. The LAN adapter in the bridge takes a copy of the frame and passes it to the bridging software. The bridging software determines that the frame is to be forwarded via Port 1, then passes the frame to that LAN adapter. This LAN adapter waits for the LAN on the right to become available and then transmits the frame onto the LAN, where it is seen by Station A. How the bridge arrives at this decision depends on the type of bridge it is. All bridges perform this basic function.