how come ribosomes made up of 50S and 30S be equal to 70S?? pls answer in simple words.. as i am poor in understanding.. :( and wat is S here... ?

The 70 S ribosome are found is prokaryotes and 50 S & 30 S are its two subunits where S refers to sedimentation coefficient i.e. how fast the ribosome move in a centrifuge. It should be noted that 50 S and 30 S are not added to make a ribosome. It is named as 70 S ribosome because it moves slowly in a centrifuge, 

Whereas ribosome in Eukaryotes is 80 S as it moves faster then the prokaryotic ribosome in a centrifuge. Hence the name of the ribosome is not based on the subunits but it is based on the speed with which they move in a centrifuge.