What does it mean if iCloud is locked on an iPhone?

I have recently been looking at iPhone 5cs on eBay, and have found some pretty decently priced ones. However, some really cheap ones will say that everything about them is great, but iCloud is locked. I was wondering, what does that mean? How will it affect my iPhone? How could I unlock iCloud? Any further information about a locked iCloud is greatly appreciated.

When the iCloud lock is on, it means the phone can't be used until the account locking it is unlocked by the original owner.

That typically means the phones are stolen, but ok, I'm sure it does happen sometime that someone has forgotten their password, lost the linked email account, etc.

Those phones are being sold for two reasons,
A) People don't understand what the lock means, or vaguely do but think it's something that can be bypassed (note: it can't).
B) People buy locked phones for spare parts, they can salvage the screen, etc.