A render-blocking resource is an asset (a <script> without defer/async, or a <link rel="stylesheet">) that the browser must fetch + parse + execute before it's allowed to render the next frame of HTML. While they're outstanding, the user sees a blank screen.
Lighthouse's "Eliminate render-blocking resources" audit lists every blocking asset on the critical path with the time saved if it were deferred. Common offenders: synchronously-loaded analytics tags, web fonts loaded without preload, and CSS files that the page uses for below-the-fold styling.
Fix by adding defer or async to non-critical scripts, inlining critical CSS in <style>, loading the rest via <link rel="preload" as="style" onload="this.rel='stylesheet'">, and moving third-party tag managers behind a user interaction.