What does it mean if i buy a o2 locked iphone 5 off ebay?

I was planning on staying on my o2 contract but switching my phone to an iphone by buying one and getting a nano sim card that o2 would give me for free. I have found an iphone 5 for over £100 cheaper (compared to apple price) on ebay but it says o2 locked instead of unlocked. Does this mean that i have to have an o2 contract (which i have so i should be okay i think?) or something else? Someone please explain to me?

Yes it just means that it is locked to the O2 network. You can be a pay as you go customer or a pay monthly customer and it will work as long as it is with O2. If your still unsure, just send the seller a message on ebay with your concerns.