Render-blocking JavaScript refers to scripts that delay the browser from rendering the page's visible content until the JavaScript has fully downloaded and executed. This increases the time it takes for the page to become interactive (affecting metrics like First Contentful Paint and Time to Interactive). You can fix this by: 1. Deferring JavaScript Loading: Use the defer attribute on non-essential scripts so they load after the main content. 2. Asynchronous Loading: Use the async attribute for scripts that don't depend on the page’s rendering order. 3. Minimizing and Combining Scripts: Minify and combine JavaScript files to reduce the number of requests and improve load time.