What is a treatment?
Internet Filmmakers' FAQ
A treatment is a narrative version of the story in a script, presented in story format, describing the main action with little or no dialogue. Many writers use treatments as a way of fleshing out their stories in narrative format to map out the flow of the action across the major scenes in the script before they start work on the dialogue. This way the writer can be sure that the story flows correctly before they commence the actually script. Most treatments are no more than 2-5 pages.
Also, check out The Treatment Toolbox article at Screentalk.biz.