In Joomla! a Section is a collection of Categories. It is the top level in the hierarchy Sections -> Categories -> Articles. For example, a website might have Sections called "Animals" and "Plants". Within the "Animals" Section, the website might have Categories such as "Birds" and "Mammals".
Sections are maintained using the Section Manager (help screen) which can be reached in the Administrator (Back-end) by clicking on the Content menu, then the Section Manager menu item.