From MEPIS Documentation Wiki
MBR -- Master Boot Record, it's typically the first physical sector (first 512 bytes) of the harddisk. When a computer boots it looks there to load the OS.
Directly from the Wikipedia page linked above:
Editing/Replacing MBR Contents
Though it is possible to manipulate the bytes in the MBR sector directly using various disk editors, there are tools to write fixed sets of functioning code to the MBR. Since MS-DOS 5.0, the DOS-mode program fdisk has included the (undocumented, but widely used) switch /mbr, which will rewrite the MBR code. Under Windows 2000 or later, the Recovery Console can be used to write new MBR code to a hard disk using its fixmbr command. Under Windows Vista and Windows 7, the Recovery Environment can be used to write new MBR code to a hard disk by clicking on Command Prompt and typing:
Some third-party utilities may also be used for directly editing the contents of partition tables (without requiring any knowledge of hexadecimal or disk/sector editors).
In Linux, ms-sys may be used to install a standard MBR. The GRUB and LILO projects have tools for writing code to the MBR sector, namely grub-install and lilo -mbr. The GRUB Legacy grub interactive console also has commands to write to the MBR. dd is also a commonly used POSIX command to copy or overwrite any sector, MBR included.