Bootloader-unlock describes a feature which supports a controlled break of the trust-chain of the device, so telling the bootloader that it should continue executing the bootshell even if the signature check has failed.

In this state the OS should continue to boot despite of this state, and applications should gracefully handle such a condition.

The crucial parts of this are also not part of AOSP, it relies heavily on the chipset manufacturer and the OS-implementation of the device-vendor.