In short: A website can put structured data on their web pages and that data may be used in rendering the page in search results, voice queries, etc.
For instance a local business could put markup that indicates the business’s hours of operation. Google may use that information to display whether the business is opened or closed at the time it shows up in a user’s search results.
Google doesn't guarantee that structured data will be rendered in rich snippets. And some features, like local data require explicit opt-in.