The concept is not too hard to understand; it occurs in several processes that occur on Earth in terms of heat transfer.  The ocean water at the poles is some of the coldest water on the Earth.  Since it is very cold, it is also some of the densest water on Earth.  This colder, denser water is also influenced by differing levels of salinity, or salt, dissoloved in the ocean water.  So this very dense, cold ocean water pushes its way towards the equator, where the Earth receives more direct solar radiation, so the water is warmer.  Being warmer, it is also less dense.  The less dense water rises to the surface, and is continually replaced by the colder, more dense polar water.  The less dense, equatorial water gradually replaces the polar water on what can best be described as a huge ocean water conveyor belt.  The exchange occurs much the same way convection heat transfer occurs in the atmosphere.  Scientists know this oceanic water migration occurs because they have tracked it with the aid of satellites and thermographic imaging equipment.