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.
Einstein developed the E = mc squared formula.
Yes.
M = mass
1905
Relativity.
Einstein developed the E = mc squared formula.
Yes.
M = mass
1905
Relativity.
e=mc2 (e equals mc squared)
No, not as science currently understands.
yes
PET scans
Albert Einstein.
C = speed of light
yup