What does the padlock symbol on my MacBook Pro mean?

that padlock indicates the Mac has a firmware password.

please see Set a firmware password on your Mac - Apple Support

if you don't remember the firmware password, the only way to get past it would be to bring the Mac to your local Apple Store and have them remove it. but please note, you will need your original proof of purchase that must be in your name, otherwise they likely will not help you remove the firmware password.

That's the symbol for a Firmware Password. It doesn't usually ask for that on a regular boot, even if it is set - were you trying to access Recovery mode?

Doesn't really matter, I guess. The simplest way to deal with it will be to make an appointment at an Apple Store or Apple Authorized Service Provider.

I'm sorry you didn't like the answers you've already received, but NO. if you cannot remember a firmware password, there is nothing that we can do to help you with that. and there is nothing you can do yourself short of bringing the Mac to an Apple Store. sorry.

At least if you bring it in they might be able to remove it. There are newer zero-trust password configurations for Filevault and Advanced Data Protection where - if you forget the password and don't have it copied to a safe place, like a piece of paper in a safe - there is no chance at all of recovery.

Every piece of security has pluses and minuses.