Hello there.
Activation Lock, introduced in iOS 7 is a anti-theft technology that prevents other people from accessing your device without permission.
The original Apple ID and Password is required to activate the phone again. For more information about Activation Lock, please visit this website:
https://support.apple.com/en-nz/HT201365
Unless you have the contact information of the original owner, or seller there is no way to unlock your iPhone. Please let me know if you have any questions or comments.
Vis