A bifurcation buoy is used to mark the point where a channel divises in to branches.You may pass this buoy on either side (port or starboard side) when moving upstream. It's coloured with red and green bands. The main or preferred channel is shown by the color of the top band.
Bifurcation buoys are used at junctions (where one channel splits into two channels). Naturally, one of the two channels will be wider, deeper, and safer; it is the main channel. The purpose of the bifurcation junction buoy is to indicate which channel is the main channel.