iCloud locked iPhone means your iPhone is linked to an Apple ID. This Apple ID has complete control over the files you have on that device. It can also block you from using the iPhone anytime through Find My iPhone Activation Lock.

If the iCloud ID associated with your iPhone is yours, you just have to login with your Apple ID details. 

However, if the iCloud ID is NOT yours, you have to remove and deactivate that account in order to use your iPhone.

To get your Activation Lock or iCloud Lock removed and deactivated, we recommend this solution:  

eBook for Removing iCloud Lock from iPhone  

This eBook is a Do-It-Yourself Tutorial which contains all legal methods that can be used by ANYONE to proceed with a Legal and Official iCloud Removal Request from Apple and/or Original Owner.  

It also includes ready-to-use templates for real case Scripts and Communication between Apple and Original owner's that will increase the chances for removing the iCloud Lock from your iPhone/iPad.