iCloud Activation Lock is an iPhone security measure which makes it harder for anyone to use or sell your iPhone, iPad, or iPod touch if it's ever lost or stolen.
The passcode needs to be guarded safely by the owner of the iPhone as this is the only way to remove the iCloud Activation Lock. If you cannot remember the passcode, you will need to contact Apple for assistance.
To know if your iPhone has this locked enabled, use the doctorSIM iCloud Activation Status Check: