https://docs.magento.com/m2/ce/user_guide/catalog/categories.html

Categories[カテゴリ]

Before you add products to your catalog, you need to establish the basic category structure of your catalog. Every product must be assigned to at least one category. Categories are usually created in advance, before products are added to the catalog. However, you can also add categories “on the fly” while creating a product. The category structure of the catalog is reflected by the main menu — or top navigation — of the store.

商品をカタログに追加する前にはカテゴリ構成を作る必要があります。すべての商品が少なくとも一つのカテゴリに属する必要があります。通常は商品を追加する前にカテゴリを作りますが、商品を作る際にカテゴリを追加することもできます。カテゴリは、メインメニューひゃトップナビゲーションに影響します。

Category Tree

• Controls

  FIELD	DESCRIPTION
  Add Root Category	Creates a new root category.
  Add Subcategory	Adds a subcategory below the current category or subcategory.
  Collapse All / Expand All	Either collapses or expands the category tree.
  Delete	Removes the current category or subcategory from the tree.
  Save	Saves any changes made to the category.