Nesting is the process of placing one sequence inside another sequence. There are many reasons why this technique is useful. This article highlights several of them, including ways to fix problems when your nested audio does not appear.
Steve Gagne sent in a question that sparked this tutorial. He was dissolving between chroma-key shots and in the middle of each dissolve, the foreground clip color-shifted to green. He thought this could be fixed using nests, but needed some help in figuring out how.