Activation lock iphone or iPhone activation lock can also be called: Apple ID Lock, iCloud Lock and Find my iPhone Lock. whatever you want to call it. It is an anti theft feature that was put in by apple to protect the theft of Apple products.
What it does is; if you have an iCloud or Find my iPhone signed into your phone and someone steal your phone and try to reset it, it will take the person to Activate iPhone page, requesting for your iCloud or Find my iPhone ID and Password without which the phone will be useless.
How to Check activation lock iphone
==> visit http://www.icloud.com/activationlock
==> Enter the iPhone IMEI or Serial Number
==> Enter the code in the image and Click Continue
If it shows you that iPhone Activation Lock is ON, that means you cannot reset the phone without having the Apple ID and password.
If it shows you that its OFF that mean you can reset the phone and its not going to ask for any iPhone activation lock ID and PASSWORD.