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Comes down to units used.
The standards are usually mass in kilograms and velocity in m/s thus the units of m x c^2 translate to the energy unit kg . m^2 / s^2 called a Joule.
In the case of a hydrogen atom of mass 1.67262158 × 10^(-27) kilograms and the speed of light as 299 792 458 m / s you get:
(approx) 1.503x10^(-10) Joules.
To put in perspective 1 Joule is equivalent to lifting an apple straight up 1m.
The Sun, being big (relative to us), burns through about 600,000,000 tonnes of hydrogen converting it into about 596,000,000 tonnes of helium every second via a nuclear fusion process. That short fall of about 4 million tonnes is what has been converted to energy via E=mc^2 or in joules about 3.595x10^(23)
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