Ip Spoofing is a method that can be used to distribute unique IP addresses for each virtual user. This can only be used if you have a pool of reseverved IP address that are in your host file and LMHost file, d by windows networking.
A example would be: You have 100 VU's and 100 unique IP addreses. Each VU would have a unique IP as would real users that are accessing your web server, rather then if you didnt spoof, each user would be using the same IP, due to werever you run your script, that IP address would be used in the payload. Hope this helps