Here's a breakdown:

* Pycnocline: A layer in a body of water where there is a rapid change in density. This change is usually caused by a difference in temperature and/or salinity.

* Density: A measure of how much mass is contained in a given volume. Denser water sinks, while less dense water rises.

Think of it this way:

Imagine a swimming pool with a layer of warm, fresh water on top and a layer of cold, salty water below. The pycnocline would be the boundary between these two layers. The water above the pycnocline would be less dense, and the water below the pycnocline would be denser.