It means that there is a certain equivalence between matter and energy. For example, every amount of energy has an associated mass. c2 (the square of the speed of light) simply is the conversion factor. In SI units, that means that 1 joule of energy has an associated mass of 1 / (9 x 1018) kilograms.
Yes.
M = mass
1905
Relativity.
e=mc2 (e equals mc squared)
No, not as science currently understands.
C = speed of light
PET scans
yes
Albert Einstein.
yup
Energy = mass x speed of light(squared)