What does the padlock at the top of the Mini I pad screen mean?

If the padlock symbol appears at the upper right of the screen, the small padlock icon indicates that the screen Rotation Lock is active.

To enable or disable Screen Rotation, you simply need to access the Control Centre. Swipe downwards from the top-right corner of the screen - then tap the Screen Rotation lock icon:

You unlock it as normal, with your passcode (or Touch ID if equipped).

Unless you're talking about some other lock function, in which case the icon wouldn't be at the top of the screen.

A padlock at the top of the screen on any iOS or iPadOS device indicates that the device is locked.