What does it mean when an iphone is locked?

So i was looking on ksl.com/classifieds for an iphone (dont answer about that) and i saw an iphone and it said " factory unlocked iphone) my pa told me it meant that if you get a locked one then you have to buy an unlocking kit… Just wondering what it means.

Factory unlocked means you can use the phone with any carrier such as AT&T, Verizon, etc.

It means that you can purchase it without registering it with a service provide (Plan) such as AT&T,

If you plan on using it as a phone though you will still have to get a plan from a service provider.

It means that you have to unlock it to get into the phone to be able ot use it

Factory unlocked means any network will work as that is what my iphone was like of trade me.

Locked iPhone is nothing but it is restricted to networks i.e you cannot access with all carriers. There is many ways to unlock iPhones. But the best among them is remote unlocking. You can get this service from any vendors online like http://www.wickedunlock.com/apple/rs1wp8.Once unlocked you can use any carrier to it