Lazy loading is a design pattern used to optimize web and mobile applications by initializing critical interface objects first and rendering noncritical items later, which enhances efficiency and user experience. In the context of React.js, lazy loading can be implemented using React's built-in features: React.lazy() and React.Suspense. React.lazy() allows for dynamic imports to be rendered as regular components, facilitating code-splitting by loading components only when needed, thus reducing the initial bundle size without cutting down the code. React.Suspense, on the other hand, is used to provide loading indicators while waiting for lazy-loaded components to render, ensuring a seamless user experience. The tutorial explores how these features are applied in React, including route-based lazy loading and server-side rendering considerations, offering practical insights through a demo app. Overall, the integration of React.lazy() and React.Suspense simplifies the process of code-splitting and lazy loading, significantly improving React app performance and user interaction.