Thymeleaf is a modern server-side Java template engine for web and standalone environments. It allows developers to create elegant, well-formed HTML5 templates that can be accurately displayed in browsers and also used as static prototypes. Thymeleaf's key feature is its natural templating approach, meaning that templates can be opened in a browser and displayed correctly as static HTML pages, even without a server. This facilitates collaboration between designers and developers. The 'thymeleaf' file extension typically represents a template file written using the Thymeleaf syntax. These templates contain HTML markup interspersed with Thymeleaf-specific attributes and expressions that are evaluated at runtime to dynamically generate HTML content. Thymeleaf integrates seamlessly with Spring Framework, making it a popular choice for building web applications in Java. It supports features like variable expressions, selection expressions, message expressions, link URLs, fragment expressions, and more, enabling developers to create dynamic and interactive web pages.