It's information in an IP header that allows the source host to dictate the path the packet uses to get to the destination rather than leaving the path to be determined by intermediate gateways. This could allow a source to go around security devices that are typically in the path between source and destination.
Cisco routers normally accept and process source routes. Unless a network depends on it, source routing should be disabled.
Source routing is a technique whereby the sender of a packet can specify the route that a packet should take through the network. As a packet travels through the network, each router will examine the destination IP address and choose the next hop to forward the packet to. In source routing, the "source" (i.e., the sender) makes some or all of these decisions.
Reason for disabling: Attackers can use source routing to probe the network by forcing packets into specific parts of the network. Using source routing, an attacker can collect information about a network's topology, or other information that could be useful in performing an attack. During an attack, an attacker could use source routing to direct packets to bypass existing security restrictions.
Use the 'no ip source-route' command to disable IP source routing on the router. Refer to your router documentation for specific instructions.