What does it mean to have an unlocked iphone?

I have been shopping for an iphone 6s and I found one for cheap but its not "unlocked" what does that mean?

Unlocked phones can be used on any carrier in the world. Locked ones can only work on that 1 carrier. It usually means it's stolen and locked to iCloud. Which means that unless you have the password from the person from whom it was stolen, it's an expensive paperweight.

It usually means it's stolen and locked to iCloud. Which means that unless you have the password from the person from whom it was stolen, it's an expensive paperweight.