When you're looking at the structured data reports for your site, you'll notice that the information will show three numbers in Google's search console. Pages with Errors, Pages with Warnings, and Valid Pages. To put it simply: Errors mean Google is likely to disregard your structured data for that page as a whole, while you can ignore a warning, and Google can still parse the rest of the data. Errors are things like data of the wrong format being specified or missing a "critical" piece of data. Generally, errors only refer to essential parts of data, like the product's price, while warnings will more likely refer to optional data, like the ISBN of a book. Warnings are when optional fields aren't populated. One typical example is in recipes. One of the most popular recipe plugins for WordPress is WPRM, WordPress Recipe Maker. WPRM has a field for nutritional information, but the free version of the plugin doesn't allow you to add nutritional information. So, the section is there, but you can't add data to it, leading to a warning in your dashboard that says "Missing field 'Nutrition.'"