According to the example in the article "Why Google Went Offline Today and a Bit about How the Internet Works", we can see the fact that the Internet is a system built on trust.
AS: the Internet is a collection of networks, known as "Autonomous Systems" (AS). Each network has a unique number to identify it known as AS number.
BGP: the networks are connected together by what is known as Border Gateway Protocol (BGP). BGP is the glue of the Internet — announcing what IP addresses belong to each network and establishing the routes from one AS to another. An Internet "route" is exactly what it sounds like: a path from the IP address on one AS to an IP address on another AS.
BGP Trust model: BGP is largely a trust-based system. Networks trust each other to say which IP addresses and other networks are behind them. When you send a packet or make a request across the network, your ISP connects to its upstream providers or peers and finds the shortest path from your ISP to the destination network.
"Route leakage": the route has "leaked" past normal paths. Unfortunately, if a network starts to send out an announcement of a particular IP address or network behind it, when in fact it is not, if that network is trusted by its upstreams and peers then packets can end up misrouted.
Example: someone at Moratel likely "fat fingered" an Internet route. PCCW, who was Moratel's upstream provider, trusted the routes Moratel was sending to them. And, quickly, the bad routes spread.