Sidekick LX 2009 unlocked for AT&T?

So I'm going to order a Sidekick LX 2009 phone fromOverstock.com. It says it's "unlocked"? I'm not really sure exactly what this means. As you probably already know, the Sidekick is a T-Mobile phone, but I have AT&T. Does this mean I won't be able to get all of the features on it? Will I still be able to call, text, use internet, etc.? Thanks!! <3