The Master Boot Record (MBR) is the first sector of a hard drive, and it contains the boot code and a partition table that describes the layout of the disk. When you install a Linux operating system, the installation process typically creates a bootloader such as GRUB (GNU GRand Unified Bootloader) that is used to boot the system.
By default, the Linux installation process will usually install the bootloader to the MBR of the hard drive. This means that when you boot your computer, the bootloader will be the first thing that runs and it will be responsible for loading the operating system.
If you have multiple operating systems installed on your computer, the bootloader will usually allow you to select which one you want to boot. For example, if you have both Windows and Linux installed on your computer, the bootloader will display a menu that allows you to choose which one you want to boot when you start your computer.
So, in short, when you install a Linux operating system, the bootloader (such as GRUB) is typically installed to the MBR of the hard drive, which means that it will replace the default bootloader that was previously installed there (such as the Windows bootloader).