It would be easier to explain this if you play mafia wars (or any game that includes intra-game exchange of gifts/items between members) but the actual scripting doesn't require any knowledge of the game.
Mafia Wars is a game on Facebook (and myspace, yahoo etc.) that among other things allows its players to send gifts to each other. Now because different people end up with more things of one type and are short of things of another type, the players have evolved a trading dynamic. Where x number of Item 1 is traded for y number of Item 2. A simple marketplace. And with this territory come scammers. Essentially a player who doesn't go through with his end of the deal. good player sends item to bad player. bad player removes good player from mafia and blocks him for ever contacting him through his facebook profile.
This has given rise to my group, among many others, that lists these scammers (identified with their facebook profile ids) as they are reported every day. Now over time the list has grown and it has become difficult and unrealistic to expect people to know who is on the list and who is not.
The user will click the bookmarklet when he is on the facebook profile of the person he is about to trade with. When clicked the script will pull the facebook profile id from the current page, and then scan a text file located on a third-party server and return with a pop-up box message that reports if the id is listed as a scammer or not.