When you create categories in your online store, they contain no products. So, you'll need to add some to make the structure work.
If you already have products in your store catalog, add them to a category on the category details page. For this purpose:
Locate a category on the category listing page of your store's Admin area (Catalog > Categories) and proceed to the category details page.
In the Products tab of the category details page:
Click the New product button to create a new product that does not exist in your store catalog yet. The process will be the same as described in Creating a New Product.
Click the Add product button if you already have products in your store catalog. The "Select products from the list" popup will open.
Select products in the popup and click Add products.
That's it! Now you have products added to a category.
If you don't have products in your store catalog yet, you can use import facilities to upload products and assign them to categories at once.
If you need to relocate products between existing categories, use the bulk edit tool.