Master Boot Record (MBR) also known as Master Book Sector is most important sector of your Harddrive which contains the vital information required for your PC to boot. MRB contains the instructions and information for BIOS on how to proceed with the boot process and load the operating system.
Like files and folder can corrupt for any reason similarly your Master Boot Record may get corrupted making your computer unbootable. If you are attempting to install new os in dual boot mode then it’s good to have backup of MBR. Here is nifty tool MBRFix for this purpose.
MBRFix lets you to fix corrupted Master Boot Record as well as create new MBR.
Screenshot of MBRFix in Action