What is Bootloader?

A small program that runs first at power-on, verifying and starting the main firmware or updating it with new firmware.

Bootloader in the explanations

A bootloader is a small program stored at a fixed location in flash that runs first at power-on.
What is the role of a bootloader in an embedded device?
If the new version fails to boot, the bootloader reverts to the original slot.
In devices updated over the air (OTA), why is flash divided…
In secure boot, immutable ROM code verifies the first bootloader's signature, that bootloader verifies the next stage or the OS kernel, and so on, forming a chain of trust.
Which describes secure boot?