According to CoreStreet, their free "SpoofStick" download is a simple browser extension that helps users detect spoofed (fake) websites. A spoofed website is typically made to look like a well known, branded site (like ebay.com or citibank.com) with a slightly different or confusing URL. The attacker then tries to trick people into going to the spoofed site by sending out fake email messages or posting links in public places - hoping that some percentage of users won't notice the incorrect URL and give away important information. This practice is sometimes known as "phishing."
SpoofStick works by clearly displaying a toolbar with the high-level domain of the site being visited, giving users the information they need to avoid being fooled. While a site may look like one created by Citibank, if SpoofStick does not say "You're on Citibank.com" then it's not a real Citibank site.