Astro is an open source web framework designed for content-heavy sites (marketing sites, blogs, documentation, e-commerce), which by default generates HTML pages without sending JavaScript to the browser, for pages that load very fast.
Its key principle is the islands architecture: the page is static, and only the components that need interactivity (a carousel, a form, a cart) load their JavaScript, independently of one another. These islands can be written with React, Vue, Svelte or other libraries, in the same project.
Astro can produce a fully static site or render pages on the server. It is often chosen for marketing sites where performance and search ranking matter more than the interactivity of a full application.
Why it matters when hiring
Astro is quick to learn for a front-end developer comfortable with HTML, CSS and a framework such as React or Vue: there is no point making it a hard requirement. The real topic is a culture of web performance and SEO. In interviews, ask how the candidate improved a site's load time and how they measured it.