The stock is the quantity of products available for sale. The stock is set up when you create your products. It can be limited or unlimited. You can consult the stock status of any product at any time from your product list in your back office, and modify it in the edit page of your product.
This is another example of how to use the Import/Export add-on. As an e-merchant, you may need to update the stock of your products on a regular basis, for example when you receive your stock, or following a physical sale.
To update the stock of your entire catalog, you just have to enter the values of the "variant-stock" column of your .csv file, then import it into your backoffice.