Southbound means traveling or moving toward the south. When a train is southbound, it's heading in a southern direction. When traffic reporters mention the southbound lanes of a highway, they mean the side where cars are going south.

You might hear someone say they're taking the southbound bus to get downtown, or that geese fly southbound in autumn as they migrate to warmer climates for the winter.

Notice that southbound doesn't tell you the final destination, just the direction of travel. A southbound airplane leaving Boston might be going to New York, Florida, or even South America. All that matters is that it's moving south. The opposite is northbound, and there's also eastbound and westbound for the other directions.