If you don't wish to track product stock you could leave Quantity empty in Main tab. Refer to Search Product on how to open up product for edit. You do this either because you want customers to place order regardless of the stock or allow them to place back order items or because the product stock is actually unlimited such as downloadable digital products.

If you wish to track product stock and block customers from placing order when product is out of stock. Enter proper stock number in Quantity field in Main tab. Refer to Search Product on how to open up product for edit.

If your product has attributes such as Color, Size etc and you wish to set stock at attribute level, you could set Quantity empty in the product Main tab and set proper stock number in Quantity column in Attributes tab. Then the stock is tracked solely by attribute quantity. If one attribute value is out of stock this value won't be displayed when customers try to choose the attribute.

If you wish to hide products when they are out of stock, go to Store -> Configuration and uncheck "Show Out of Stock Items".