What it means — Apple
If your Mac is using a firmware password, the lock icon appears when you try to start up from another disk or volume, such as an external drive or macOS Recovery.
What to do — Apple
- 1 Enter the firmware password to continue.
Read the full article for pictures and model-specific details: If your Mac doesn't start up all the way ↗
Related
- Mac says a software update is required to connect to iPhone or iPad
- Flashing question mark at Mac startup
- Circle with a line through it at Mac startup
- Globe with a warning symbol at Mac startup
- Activation Lock or system lock PIN at Mac startup
- Image of a mouse or keyboard at Mac startup