The MBR contains the map telling the beginning and end of disk partitions, what type of format each uses, whether it's visible or hidden, etc.
It is created when you format your disk. It is modified when you create or delete partitions.
There are several free utilities you can use to explore the MBR. They all give a warning that you can ruin your disk file system (if you press the key that saves changes which you make).
I have used MBRTOOL on my vintage 2003 Windows XP machine. I rebuilt my MBR once, with the idea that it would solve a serious problem. MBRTOOL installed its own startup code, overwriting the previous code which had previously allowed me the option of re-installing all XP system files (which can sometimes solve a problem).
The utility does not run under Windows. I have to boot to DOS from a floppy or bootCD. Then run the utility.
Such utilities can be downloaded free. Or they can be found on free boot CD's including the Emergency Boot CD and the Ultimate Boot CD. Also on 'PC wizard' type cd's which can be purchased on Ebay for a small price.