In short, anti-spoofing was designed to stop a computer from faking another computer's IP address. Spoofing is generally used by viruses or zombie PC's to hide their origin so that they can perform malicious activities like spread themselves or DDoS someone without being caught.
On a specific Interface properties for a gateway, you have two options sections: Topology and Anti-Spoofing. Identifying the topology of an interface is listing all the IPs that could originate from behind that interface. The Anti-spoofing defaults to blocking all IPs that you don't define as being in the topology.
An example would be, you have a DMZ of 10.1.1.1. A computer is infected and wants to serve malicious code out to unsuspecting victims, that computer sends out packets saying "I'm
Google, here's your malicious webpage". What Anti-Spoofing will do is say,
Google aka 192.168.1.1 doesn't exist on this interface so I won't allow this traffic to pass, thus saving the victim from being infected. Now you're the hero.
HTH