What it means — Apple
If your previously owned Mac shows an Activation Lock window that asks for someone else's Apple Account, the previous owner may need to remove Activation Lock for you.
Your Mac might ask for a PIN code when it has been remotely locked using Find My because the owner thinks the Mac is lost or stolen.
What to do — Apple
- 1 Enter the four-digit or six-digit passcode to continue.
Read the full article for pictures and model-specific details: If your Mac doesn't start up all the way ↗