Activation Lock is intended as a theft deterrent. The Activation Lock password is the Apple ID associated with the device. That usually takes the form of an email address, though other formats are possible. The Activation Lock prompt will display a hint; showing a letter and part of the email address. If you have forgotten the Apple ID or its password, here are links to what Apple suggests:
If you forgot your Apple ID password - Apple Support
If you forgot your Apple ID - Apple Support
How to use account recovery when you can’t reset your Apple ID password - Apple Support
How to request access to a deceased family member's Apple accounts - Apple Support
Contact Apple for help with Apple ID account security - Apple Support
If you’ve forgotten it all and cannot recover access to the Apple ID, which means you can’t get around the Activation Lock, then you’ll need the purchase receipt and a discussion directly with Apple Support.