What is a bus bar in an electrical switch board?
I think Wikipedia can explain what a bus bar is, better than I can. But in general terms. It's a strip of conductive material where other conductors can connect, to safely interconnect all the conductors.  

If you look in an empty single split phase electrical panel, you'll see 3 or 4 bus bars. Two ungrounded (hot) bus bars running down the middle, one grounded (neutral) bus bar off to the side or at top/bottom, and one grounding bus bar off to the side or at top/bottom.