The "In Stock" and "Full Stock" fields are linked together to help you manage your inventory. "In Stock" is how many products you have available right now for purchase, and "Full Stock" is how many you started with in the first place. If you sell exclusively online, these will stay the same, and you'll only need to tweak them when you get new stock in. If you also sell at events or on other sites, you can manually update these anytime. After you've made sales, these numbers will tell you how many you've sold via Storenvy and how many you have left in stock. If you show availability on your product pages, the stock levels will appear to shoppers.