Let's break this down - first, "quantity updates". When a sale occurs on one of your connected channels, the quantity of the item needs to change on your other channels to reflect what you have available. For example, if you sell 4 copies of the Captain America blu ray on eBay, then you need to account for that lack on your other channels. Similarly, if you get a new shipment in of a certain SKU, you're going to want to update your channels to put more quantities available for sale. The "real-time" part of this "real-time quantity update" feature means that your other channels are updated in real-time - that is to say, not only once per day, but frequently throughout it.
SkuVault can connect to your channels in a variety of ways - through a channel management platform like ChannelAdvisor, via shipping software like ShipWorks and ShipStation, through an eCommerce platform like Shopify, Magento, or Volusion, or even through some channels directly, like Amazon, and all the various international versions of eBay, and through a variety of other platforms.
Take our earlier Captain America blu ray example - say you have 5 total copies in your warehouse, and you just sold 4 out of the 5 you had listed on eBay. Say you also had those 5 blu rays listed on Amazon - if your quantities aren't updating in real-time, then you are on the fast track to get hit with out of stocks. If more than 1 sale comes in before you get around to updating your quantities, then you are in out of stock territory. Say the reverse is true, and you just received a shipment that contains 5 new copies of that blu ray, and you don't get around to updating your quantities till that night, or the next day - in this example, you've missed out on potential sales, since you have the product on hand, but your listings have been empty. Out of stocks and undersells lead to all sorts of problems - lower feedback rankings, lower product listing placement in search results, and, ultimately, lower sales. Real-time quantity updates helps to greatly mitigate this problem!