Source routing is the act of inserting a source IP address in to the TCP/IP packet headers using an address from inside the network you are attacking. In other words, make the packet appear as though it originated from inside your own network, when in fact it is actually coming from someplace outside your network. The idea is that your network should obviously trust itself, and take action on the packets as though they were from a trusted source. This way, packets that would normally be denied access are easily sent in to the network - unless you've eliminated this probability.