what is the effective mass in solid?

Answer 1

Mass is a measure of "how much matter there is in an object." In the case of a solid, the amount of mass is how much of the substance is there.

Answer 2

For many purposes, the effective mass of a solid can be the same as the actual mass but concentrated at the center of gravity - the point where there is equal mass on opposite sides in all directions.
This doesn't work for rotation, but it does work for gravitational attraction, balancing and other aspects.