The Master Boot Record is a critical part of a computer, the MBR contains the Partition Table, and tells the computer what partition to boot into. The Partition Table contains documented information about each partition on the hard drive [Volume, Type, Format etc.]
Unfortunately, as the Master Boot Record is so vital in a computer, it is obviously a target for viruses, especially trojans and ransomware. These ransomwares will write their own code to the Master Boot Record, normally asking for a code to unlock the computer. An example video is below, of Trojan.MBRlock. This specific Ransomware is written in Russian, and asks the user to call a specified phone number to retrieve a code.