A coplet is a COcoon PortLET. Coplet is a component in the Cocoon Portal framework (deprecated) and in the Cocoon Portal block (new). The Cocoon web page on portals (http://cocoon.apache.org/2.1/developing/portal/) gives details.
This page doesn't seem to be much more than a definition and a link. Why bother?
It was written because the author had a hard time finding out what a coplet was, through existing means. Hope it helps you.