What does it mean when your IMEI is unlocked on iphone 4s?

My dad found an iphone 4s and it was disconnected (no service).so he restarted it with itunes.today we checked the IMEI # online. It said it was unblocked. What does this mean? Btw it is from at&t

Added (1). But does this mean that i can go to like tmobile or something and connect it there? Or do i still have to get it factory unlocked?

Added (2). Tmobile costs too much for a monthly pay. Are there any other ones that will connect an at&t phone with about 50 dollars / month

That means its factory unlocked. Which means you can use any carrier that uses sim cards. You can also use over seas sim cards

It means the iphone has not been reported lost or stolen by the original owner

EDIT---but does this mean that i can go to like tmobile or something and connect it there? Or do i still have to get it factory unlocked?
--- you still need to jailbreak and unlock it via a cydia app (simple google search)

tmobile costs too much for a monthly pay. Are there any other ones that will connect an at&t phone with about 50 dollars / month
--- Straight Talk

That means the phone is factory unlocked and now you can use the phone with any SIM card of your choice. You can go to SimpleMobile which is $50/month or StraightTalk which is $45/month unlimited everything.

Well you can just get your iPhone Factory Unlocked from http://www.officialphoneunlock.com and have no worries, no hassles at all.
It's unlocked remotely so there's no problem usually and your iPhone will also be capable to work on other carriers/countries

They just register your IMEI # into Apple's servers so you can unlock with iTunes and your iPhone gets equal to a factory unlocked iPhone.
I recommend because my friend and I got our iPhones unlocked from there