When you visit a webpage, send out an e-mail or watch a funny cat video, how does that particular piece of data find its way from one end of the Internet to the other? You may be aware that the Internet is a collection of many smaller networks, connected to each other using routers. When a router recieves a packet of information, it will check the destination address and then forward it to the next router along the way. That router repeats the process, and so on, until the final destination is reached. So, how do routers know in what direction they should be forwarding these packets? The answer is that they use routing protocols to share information with one another. Suppose that router A knows of a way to reach a certain destination. It will communicate this route to other routers. On receiving this information, router B may answer: sure, you may know of a way to reach that destination, but I know this shorter route!
The Border Gateway Protocol
To connect various different networks together, we really all need to speak the same language. The routing protocol that thus forms the backbone of the Intenet is BGP, the Border Gateway Protocol. Like many of the fundamental protocols that make the Internet work, it was not designed with much focus on security. Routers will simply accept the updates they receive from other routers, without validating the authenticity. And this is, in fact, a big problem!