Question

what is mean sea level

Answer 1

The sea level /halfway between the mean levels of high and low water. Or a measure of the average height of the oceans surface. The averge height of the surface of the sea for all the stages of the tide, it is used as datum level or reference for elevation.

Answer 2

There has to be a reference for every measurement that should work universally,so in order to measure the distances and heights of anything we need make a reference that is mean sea level.The height measured from the M.S.L is called reduced level.As the earth surface if not uniformly distributed,we will take this reference to be M.S.L and that is assumed to be constant everywhere.