In computing, a bus is a connection between multiple components which allows those components to share information, power, or instructions relating to controlling those devices.
A common example is USB. The Universal Serial Bus interface is used to connect external (and occasionally internal) devices to a computer. Multiple devices are all connected to this one interface, and the electrical signalling on that interface determines which device or devices are in communication at any point in time.
Another example is the bus internal to the computer that connects the CPU to multiple banks of RAM and other internal components. While electrically very different than USB, the concept – that of connecting multiple devices to a single communication path – is the same.