So, let's say you're on a product page for a vacuum cleaner on Walmart. Cortana can look up the price of that particular vacuum cleaner in competing stores (like Home Depot or Best Buy), while you still have that page open and it'll let you know if the vacuum cleaner is available for a cheaper price elsewhere. Think of it as a Kayak for online shopping, but built right into the browser. It might not be as full-featured as Microsoft's Personal Shopping Assistant (which lets you get notified of price changes), but at least Cortana doesn't require installing a browser extension.