Hi there!
Think of a bus as a plug socket.
And your aircraft is a big room with many sockets.
Now the difference between your living room and your typical aircraft is that each socket in the plane has a different power source (not all, but many)
Some are only powered by the battery, others only by generators or by transformers.
Also, the sockets aren't directly connected to each other. If one of your sockets in your living room has a short circuit, most appliances in that room will fail as the circuit breaker trips. Not good.
In an aircraft, a complete bus can fail (an AC bus for example) without affecting the other systems.
Of course some busses are connected but to keep it simple I won't go into that. (If you need more info go ahead and ask)
I hope that helps?
Regards,
Mark