Section refers to a collection of categories. Section is the top level in the hierarchy Sections -> Categories -> Articles. For example, in your website you can have a section Four-wheelers and within this section, you can create the categories: Cars and Trucks. In the Cars category, you can have the articles Mercedes Benz and Hyundai Santro, and in these articles you can have detailed information on the respective products.
For more information, refer to Sections.