It's a name for a certain (not necessarily electrical) topology.
Assume, you have a host unit and several guest units you want to connect. Then there are 2 main possibilities:
1) Connect every item separately, i. e. with its own cable - this is called
star topology. 100 MBit Ethernet is an example, it *needs* a central node (switch).
2) Connect every guest with the very same cables like a daisy chain - this is called
bus topology. The central heating in a multi story building is an example, because one riser line serves multiple radiators.
Of course you want to save weight on every aircraft, therefore not every unit has its own connection to battery / generator / whatever, instead the lines are extended from one unit to the next. (Well, since you wouldn't extend from something in the starboard wing directly to the outer portside, this has its limits, but its called "bus" anyway because of simplicity.) In general, most power supply layouts in electric / electronic devices are busses, not only in aviation industry.
Also data connections are sometimes buses. The disadvantage is the protocol overhead you need to tell the connected devices which unit is the addressee of a particular data packet. It can take quite a bit of extra bits and electronics in order to sort these things out. Also redundancy suffers when the cable is cut somewhere, because all units are affected at once. But obviously you can take care about these things - A380 is doing a major part of its communication via special Ethernet derivative.
If you are interested further, Wikipedia tells a bit more when you search for "star network" or "bus network". (This is where I got the images from.)