Bridge mode works in the following way:
Say you had 2 networks in 2 separate buildings and wanted to link them together.
You could run a single ethernet cable, or fibre via a media converter, or use 2 access points in bridge mode.
They are transparent to the network, as far as the network is concerned, it is just a cable link with a little packet loss and a slower speed.

This is much the same for multipoint bridges, just the one 'master' end works like a hub or switch.

For what its worth, 3 access points running in multipoint bridge mode or 1 in AP mode and 2 as clients are the same thing as far as the network itself is concerned.

Also, I do not believe bridge or repeater modes are actually specified in the 802.11b specification, this means that they *may not* work between different vendors of products.

Feel free to correct me if I am wrong.