Structured data helps search engines find out what's what on your website. Read here what structured data is
Your website contains a lot of information. That information is typically data about something. If you organize a concert, there might be data about concerts. It could be:
If you have a recipe, it might contain information about:
And if you have a webshop, it will typically contain information about
This is all data that humans can see through when reading a website (if it's presented fairly well).
But robots will also visit your website. The most important robot is probably Google's robot, GoogleBot.
It can have a hard time knowing if something is in stock, what the price of a product is and what the brand is. That's why it has been decided that if you provide the information in a certain format (i.e. if you structure your data), Google (and other products) will know the price, stock status, brand, etc.
This is what we call structured data.
There are countless. There have been many different types of structures over the years, but the most commonly used today is the one called Schema.org (which Google, Bing and Yahoo have all agreed to follow).
Here you can see a list of the data types that they have identified and created Schema rules for. For example, you can see the possibilities and requirements for a product's structured data here.
Google has created an excellent tool for testing your structured data. You can find it here.
We've been working with online marketing ourselves for decades. As the only shop system in the country, we have spoken multiple times at conferences such as Marketingcamp, SEOday, Shopcamp, Digital Marketing, E-commerce Manager, Ecommerce Day, Web Analytics Wednesday and many more.