IP source routing is a mechanism that allows the sender to determine the IP route that a datagram should take through the network. An attacker could use source routed packets to obscure their identity and location. Source routing allows a computer that sends a packet to specify the route that the packet takes.
Read the full blog post: https://calcomsoftware.com/the-policy-expert-mss-disableipsourcerouting-ip-source-routing-protection-level-protects-against-packet-spoofing/