Categories are a great way to organize your media. Click on the Category filter drop-drown at the top left corner of your media library, and click on “+ New Category” to add a new category.
To move selected files to an existing category, select the files and click on the “Add To / Remove From” drop-down to select categories. The number of files in each category is listed next to each category name. A dash indicates some of the selected files are in that category. A check-mark indicates that all selected files are in that category: