Pods are a concept that was released with the Central product by Macromedia. From an article on Breeze, it seems that the concept has grown past Central into some form of standard for Flash programming. A working definition of a pod might be:
"A pod is a SWF which contains the UI, Code and initial data for an application. This application is not meant to be independent but instead is meant to be a sub-component of a larger application. In layout terms, a webpage that has a standard header, sidebar, body and footer could be said to contain 4 pods, 1 for each section. Alternately, it can be said that each section displayed in the body section is a pod and the surrounding layout is the containing site/application."