Explain how the changes in the density of ocean water causes movement of ocean water from one place to another

The density of the water causes pressure, which causes water to move. 

It is called diffusion.
Density of the ocean is highly depending on the amount of salt in it which chages frequently because of rain, heat etc. So diffrences in amount of salt occurs in different parts of the oceans. So water in the ocean moves to equalize the amount of salt in itself.