Hi,

Meaning of reduced to sea level :- The application of a correction to a measured horizontal length on the earth's surface, at any altitude, to reduce it to its projected or corresponding length at sea level.

Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured .

Reduced here means equating and Level means Elevations. Datum may be real or imaginary location with a nominated elevation of straight zero.

Hope this helps.