What does it mean when my iphone is locked?

I bought an iphone 5 from sprint (Full price), I got fed up with sprint because one month my bill would be $60 than the next it would be 90 or 120 dollars a month. Last month my bill was $390 with no explanation on why from customer service. I have unlimited everything so there's no reason for my bill to be that high and to keep changing. I'm so fed up with sprint, that I decided to drop them and go with another phone company. I went to Verizon and they told me they won't activate my iphone because it is locked to sprints network? What does this mean? Does this mean I can't use my iphone from anyone else but sprint? This is bull **** because it does not have a sprint logo on the back, I should be able to use my iPhone with whoever I want. I payed for the phone. If I can't use my iphone with Verizon, can I exchange it for the same iphone I got that will work on Verizon, I don't want to have to pay for another phone? I think it is complete and utter bullshit that I have to buy a new phone. Can I sue sprint for locking my iphone to their company only? I payed for the phone full price and it doesn't have their logo on it, so I should be able to use my phone with anyone I want as long as I'm not in a contract, which I'm not.

When my iphone is locked it means its limited to one carrier only, you could unlock it though. Pay someone to do it or try searching> unlock iphone from sprint.

Locked means it would only work on that carrier.