Or better yet, just google it:
A treatment is a summary of a screenplay, TV show, novel or other story, sometimes in the form of an outline. It can be anything from one to ten pages or even longer in length. Treatments can be used as a tool of development, encapsulating a story's most important elements in a more manageable format, for either the writer's own creative purposes or as a vehicle for a producer to give his or her input. Or it can be a marketing tool, a summary of your story that is easier to share with producers or others who might be considering the value of your project before they decide whether to get involved (either financially or creatively or both).
You may have already finished your entire work and are now reverse-engineering a treatment for marketing purposes (or to facilitate a pitch). Or you may see it as a starting point, a blueprint for your eventual full work.