iCloud Active. for iPhone 5s I want to buy a iPhone from Mercari. It's in great condition but it says "no iCloud Active". Does anyone know what this refers to?
This is a good thing, it means that you are free to add your own iCloud account. To add more information, having it linked to someone elses iCloud account is a really really bad thing. When I used to work for Apple, we would get calls from people who thought they got a great deal but when they tried to upgrade to iOS 7, or use the phone for the first time they found out that the phone was stolen. 

Often these were bought at pawn shops.
I don't mean to say that pawn shops traffic in stolen devices, I do mean to say that they lack the sophistication to understand that the previous customer needs to unlock / sign out of their iCloud account. In addition Apple has a policy of never referring to a device as stolen, so even if I was fairly convinced that I was talking to someone who had actually stolen the phone, we left it up to law enforcement to sort this matter  out.

This led to some rather interesting situations that I can't discuss here. I will say that I had customers call who had purchased their device for a major retailer who had taken a return and resold the phone locked to the previous owners account.

For a better understanding of this feature of Apple since iOS 7, look at these links from Apple support.

https://support.apple.com/en-us/HT201365

https://support.apple.com/en-us/HT201441