Structured data is created using a predefined schema and is usually organized in a table. The schema is the outline of how the data is organized, the header row of the table is used to describe the value and format of each column. The schema also imposes the constraints necessary to make the data consistent and computable.
An example of structured data is a relational database: tables are linked using unique identifiers and the language used to interact with the data for SQL queries.
On the other hand, unstructured data can come in many forms: from web pages to emails, from blogs to social media posts, etc. It is known that 80% of the data we have is unstructured. Regardless of the format used to store the data, in most cases we are talking about text documents composed of sequences of words.
Structured data is the best way for computers to interact with information. That's why it's essential for SEO: by adding structured data to your website content, you allow Google and other search engines to understand what you're talking about and show your website among the top results in user searches.