Astro (or Astro.js) is a JavaScript framework for building multi-page applications. Whereas many other frontend JavaScript framework's main focus is building web applications, Astro is designed for creating content-rich websites, such as blogs, portfolios and marketing sites, though Astro is not excluded to these and is also a valid choice for building more complex web applications.
The main benefit of Astro is that it doesn't ship any JavaScript to the client and prioritizes running on the server when possible, making it a lot more performant and in-line with its content-focused approach.