The current copyright laws in most countries (including the United States, where IFDB is based) say that you automatically own the copyright to any original intellectual works you create, such as writings or artwork. A "copyright" is literally the right to copy. This means that, by default, you're the only person who's allowed to make copies of an original work you create.
That's the default. However, in practice, most authors and other artists would find it tedious and unwieldy if they had to personally create all of the copies of their works for sale. That's why the law lets you extend your exclusive copying rights to other people, such as publishers (and Web sites like IFDB) by granting them special permissions. Further, if you choose to grant such permission, you can make it conditional: you can attach restrictions to it. For example, you could grant permission only to specific people, or you could say they can only make copies on alternating Thursdays. This kind of conditional permission, with a list of restrictions, is called a license.
When you upload your original work to IFDB, such as when you write a game review, you're obviously making the initial copy of your work. However, in the course of its normal operation, IFDB has to be able to make additional copies: it has to display the review when someone views the game's page, for example, and it has to store parts of the review in its search index. This means that IFDB couldn't legally use the content you upload, just in terms of its technical operations, without your permission. This is why we need you to grant us a license.
Now, obviously, it's not practical for us to ask you to ask your lawyer write up a license and send it to us each time you upload something to IFDB. That would be expensive for you, and it would be impossible for us to keep track of the differences in all those licenses. So, like all "Web 2.0" sites, we provide a standard license that everyone uses for uploaded material, so that all uploaded content is uniformly licensed. All you have to do is agree to use the license we provide, which you signify simply by uploading your material. (We know that many people feel that "click-wrap" agreements like this are obnoxious, and we kind of think so too, but we're pretty sure it's the only practical way we can operate a site like this.)
This is where the Creative Commons license comes in. We could have gone the route of many other sites, which have their own licenses for user-submitted content, drawn up by their own lawyers. But we felt that we're already asking a lot when we ask you to license your material to us in the first place; we didn't also want to impose one of those endless streams of fine print that protects us at your expense, as you'll often find on commercial sites. Instead, we decided to use a well-known public license created by a neutral third party. That way you can be sure that we didn't hide any self-serving "gotchas" in the legalese. We chose Creative Commons because their licenses are well regarded for protecting the rights of authors while fostering sharing. The CC license was created by copyright law professionals, and it's been widely reviewed, so we can all be comfortable that it's fair and reasonable and legally sound.
In short, by using the Creative Commons license, we're not asking you to give IFDB any special permissions; we're simply asking you to share your work with the whole IF community.