The website can track the stock level of products. 

This is configured in the Sales Information section of Product details...


First tick the Enable Quantity check box to activate the Quantity and Minimum Stock boxes.

Quantity is the current quantity held.   This is reduced as the product is sold.

Minimum Stock is the level above which the product can sold.   If the Quantity falls below this level an "Out of Stock" message is displayed against the product and it is no longer available for sale.  (This allows you to use actual stock levels but still have a safety margin for web sales in case the stock is sold through a different channel.)