What is a Megapascal?

The megapascal (MPa) is a 1,000,000 pascals (Pa). The pascal is a SI unit that measures pressure. Pascal unit is named after Blaise Pascal and it replaced the previously used units for pressure $N/m^2$ in SI unit system. Following are equal representation of 1 Megapascal in other units.

$$1\ MPa = \frac{1000 * 1000 *N}{m^2} = \frac{1000 * 1000 *kg}{m*s^2} = \frac{1000 * 1000 *J}{m^3}$$

Where
$N - Newtons$
$kg - Kilograms$
$m - Meters$
$s - Seconds$
$J - Jules$

Wikipedia page link for Pascal

What is a bar?

The bar is a metric unit that measure the pressure. Equivalent unit Pascal (Pa) used in SI units to measure pressure. 1 bar is same as 100 kilopascals or 1 hectopascal (hPa).

Wikipedia page link for Bar
Article on Atmospheric Pressure and Gauge Pressure
Article on Fluid Pressure Calculation and Atmospheric Pressure