Back in the days of computer yore, PC motherboards consisted of lots of discrete integrated circuits. This generally required a separate chip or chips to control each system component: mouse, keyboard, graphics, sounds, and so on.
As you can imagine, having all those various chips scattered about was pretty inefficient.
In order to address this problem, computer engineers needed to devise a better system, and began integrating these disparate chips into fewer chips.
With the advent of the PCI bus, a new design emerged: bridges. Instead of a bunch of chips, motherboards came with a northbridge and a southbridge, which consisted of just two chips with very specific duties and purposes.