The previous reply described what a master boot record is.
Let me add that by itself, a master boot record is not a problem to be
fixed. it is vital to the operation of your PC.
Unless your anti-virus software said that there was a virus in the MBR, do
not mess with it. In case of a virus, let the anti-virus software try to
fix it. Most major brands (i.e., Norton and MacAfee) will have a provision
for automatically doing a repair, or will provide additiona information, or
possible send you to their website for more help.