This article is from the Ethernet FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
Transparent bridging is a method to connect two similar network
segments to each other at the datalink layer. It is done in a way
that is transparent to end stations, hence end-stations do not
participate in the bridging algorithm.
Transparent bridges are sometimes called learning bridges.
When they are turned on and receive data packets from a network
1) learn the relation between MAC address and segment/port, and
2) forward the packet to all (!) other segments/ports.
The first step in this process is essential to the "learning" aspect
of the bridge. After some time the bridge has learned that a
particular MAC address, say MACa, is on a particular segment/port,
say PORT1. When it receives a packet destined for the MAC address
MACa (from any port not being PORT1) it will no longer forward the
packet to all ports (step 2). It knows that MACa is associated with
PORT1 and will only forward the packet to PORT1.