Prerendering is a technique used to optimize the experience for search engine bots (like Google's web crawler) when building Single Page Applications (SPAs). SPAs, being fast and client-side only, can sometimes not serve all necessary information to bots, leading to crawl issues. Prerendering preloads all elements on the page in preparation for a bot to see it, rendering JavaScript and assets statically. This approach allows developers to build their sites using modern frameworks like React or Angular without relying on server render solutions. However, prerendering is not necessary for all sites, especially those with login screens or static HTML pages. It's recommended to use prerendering only when the content lives behind a SPA and needs to be crawled by bots.