What is meant by sendimentation constant "S"?And the sub units 50S , 30S also?

Sedimentation coefficient (s) of a particle determines its behaviour in sedimentation or centrifugation process. It is defined as the ratio of a particle's sedimentation velocity to the acceleration that is applied to it (causing the sedimentation). In simple words it determines how fast a particle moves and sediments in a centrifuge. 

The 70 S ribosome are found is prokaryotes and 50 S & 30 S are its two subunits. It should be noted that 50 S and 30 S are not added to make a ribosome. These are named as the 50 S & 30 S based on their speed with which they move separately in a centrifuge.