What is Boot ???

Alternatively referred to as boot up, booting is the process of powering on a computer and getting into the operating system. During the boot process, the computer will perform a self-diagnostic, also known as a POST and load necessary drivers and programs that help the computer and devices communicate. As your computer is booting, you may see a screen similar to the example picture below.
Computer Boot screen
In the above example, this screen is displaying the BIOS information, the type of computer, CPU, and memory for the computer and a prompt to enter BIOS setup.
If the computer cannot boot you may receive a boot failure, which is an error that indicates the computer is either not passing POST or a device in the computer such as the hard drive has failed.