A container category is a category that contains only subcategories rather than articles. These subcategories will normally themselves contain articles.
For example, Category:Drummers by nationality is a container category, and contains only subcategories, such as Category:British drummers. Articles on individual drummers are in the subcategories of this container category, for example British drummers are in the subcategory British drummers, or in subcategories further down the tree.
Containerization is a process of turning a regular category into a container category by splitting it using additional criteria. For example, nationality can be used as a criterion, as in the drummers example above.