Simplest way to think of backplane is to visualise it as a highway and the individual ports are exit/entry points onto the highway. So if traffic enters one port and needs to go out via another port the traffic will be sent on the highway from the first to the second port.
Not all switches have a big enough highway to support traffic from all ports at the same time. If they do have a big enough highway then the switch is said to operate at wire speed with no blocking.
Edit - apologies but forgot to mention, think of the individual packets as cars/lorries etc.
Hope this makes sense.
Jon
