If activation lock is on then you can’t erased that device and won’t be able to enter into homescreen until you have Apple ID and password to which it was locked.
So if you are planning to buy iPhones, iPad or iPod touch from another person or from a third-party, make sure to ask for the IMEI number and beware of icloud lock in it.
But if when activation lock is off then you can reset and can activate your iphone using activation sim very easily.
How to enable Activation Lock & how it works?
We mainly enable find my iPhone on our devices for more security of the devices so that another person cannot access and use the iCloud locked devices. So when you turn on Find My iPhone on your iPhone, iPod touch or iPad iCloud lock or Activation Lock is automatically enabled. Likewise, if disabling Find My iPhone on an iOS device will automatically disable Activation Lock protection. Activation Lock works with the iPhone, iPad or iPod touch with iOS 7 or later.
To enable Find my iphone, Go to Settings >> iCloud >> Find My iPhone
Here’s How to quickly check the iCloud Activation Lock status of any iOS device using Apple’s web tool.
2) Now type in the IMEI or serial number of your iPhone, iPod touch, iPad that you wish to check the Activation Lock status for.
Here’s how to find IMEI number and Serial number on iPhone?
IMEI and serial numbers are available in Settings >> General >> About.
From iTunes. Plug in iTunes and select the device.
On the Activation Screen
On the original packaging for the device
On the back of your iPhone
On the SIM tray
Apple devices has located IMEI number on the back upto iPhone 6 Plus whereas iPhone 6s does not. But you can find the IMEI number on the sim card tray for the iPhone 6s and 6s Plus. So, make sure to check out the in the different locations.
3) Type the verification code and click Continue.
If Activation Lock is enabled on the device then you will receive an “Activation Lock: On” message with a note saying that the current user’s Apple ID and password will be required before anyone else can activate and use this device.
If you wish to check another idevices Activation Lock status then Click Check Another Device.
If the previous iPhone owner has erased the device remotely, or put in Lost Mode, a message on the Lock screen should say that the device was either lost or stolen and when you wanted to restore the lost iPhone you would see the activation screen and when you try to activate it will stuck in Activation state asking for Apple ID and Password.
This tool helps a lot whether it has Activation Lock enabled when buying a used iPhone from any other person and get rid of thieves.
Last but least: Don’t forget to turn ON Find My iPhone for security and turn off find my iPhone before giving or selling it to someoneelse. Otherwise you won’t be able to reactivate the device without Apple ID and password.