Refer to figure.
Bus bars are a way of simplifying the distribution of electrical power through a network. They are thick copper bars with holes drilled in, where wiring can be attached to be split to go to other locations/consumers within a network. They can run off one or multiple sources of power, and are much simpler and lighter than individual wiring to every generator/consumer.
A short circuit is where electricity finds an easier way to flow from high voltage to low voltage. Usually, we want that electricity to flow through our consumer, which has some resistance, but if a significantly easier route exists, the amount of current running through a consumer will decrease, as an easier path exists for the electricity. If, for instance, a loose, uninsulated wire going straight to "ground" touched a bus bar, then all the electricity available to the bus bar would begin to flow through this wire, as it is the easiest path to go from high voltage to low voltage, completing the circuit. This would not only be a fire hazard, as the high current would heat all components up far too much, but would also mean that all connected consumers would not receive any electrical current, and would stop working. Think of it like a "black hole" for electricity, all the current will be sucked down the short circuit and will not feed other consumers.
To avoid this eventuality, bus bars have circuit breakers that will disconnect above a certain current. This meant that the bus bar will become isolated from the other circuits in the aircraft, and no more electricity will be lost due to this short circuit. It does, however, mean that the consumers that only take power from that bus bar will be unable to receive power for the rest of the flight.
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