A network bridge is software or hardware that connects two or more networks so that they can communicate.
People with home or small office networks generally use a bridge when they have different types of networks but they want to exchange information or share files among all of the computers on those networks.
Here's an example. Let's say you have two networks: in one, the computers are connected with cables; and in the other, the computers are connected using wireless technology. The wired computers can only communicate with other wired computers, and the wireless computers can only communicate with other wireless computers. With a network bridge, all of the computers can communicate with each other.
If you use the network bridge software built into Windows, you don't need to buy additional hardware.