Hboot, or we may call it Bootloader, is like BOIS to windows.

It is the first thing that runs when you boot up your Android device.

It packages the instructions to boot operating system kernel and most of them are specifically designed to run their own debugging or modification environment.

What is bootloader used for?

A Bootloader is the first program which executes (before the main program) whenever a system is initialized. In PC It is used to boot he OS of the system. In case of microcontrollers, a bootloader enriches the capabilities of the microcontroller and makes them self programmable device.

What is meant by reboot to bootloader?

Rebooting to system is just normal Android. Nothing special. If you boot to recovery, you can do stuff like factory reset your phone or install updates. Or you can boot into download mode (aka bootloader). The way you do this varies by phone, but you usually have to press volume down and power.