Can someone explain to me in simple terms what is relative mass and absolute mass? And how to calculate them too. Thanks.

Man X has a mass of 100 (pick a unit--gerbils.)

Man Y has a mass of 300 gerbils.
Their absolute masses are 100 and 300 gerbils respectively. Their RELATIVE mass is 1/3 or 3/1 depending upon how you compare them. Or compared to a piano in the same room with them at an absolute mas of 1000 gerbils, their relative masses are 100/1000 and 300/1000.

Look at atoms. The masses of the elements in the periodic table are masses RELATIVE to carbon which by international standards has been set at 12.01. But the actual mass of an atom is much less than that.