The packets of an internet message
All data transfers across the Internet work on this principle. It helps networks manage traffic -- if one pathway becomes clogged with traffic, packets can go through a different route. This is different from the traditional phone system, which creates a dedicated circuit through a series of switches. All information through the old analog phone system would pass back and forth between a dedicated connection. If something happened to that connection, the call would end.
That's not the case with traffic across IP networks. If one connection should fail, data can travel across an alternate route. This works for individual networks and the Internet as a whole. For instance, even if a packet doesn't make it to the destination, the machine receiving the data can determine which packet is missing by referencing the other packets. It can send a message to the machine sending the data to send it again, creating redundancy. This all happens in the span of just a few milliseconds.