Hydration errors in React occur when there is a mismatch between the server-rendered HTML and the client-side React component during the hydration process, a common issue when using Server-Side Rendering (SSR) frameworks. These errors arise because the server and client may render different HTML, often due to discrepancies in props or browser-specific elements unavailable to the server. The post explains the basics of SSR, contrasts it with Client-Side Rendering (CSR), and provides a step-by-step example of building a React SSR application to illustrate how hydration works. It also highlights scenarios that can lead to hydration errors, such as differing server-client props and invalid HTML structure corrections by browsers, and offers strategies to mitigate these errors, like using a custom hook with useEffect to ensure consistency between server and client rendering. The post emphasizes the importance of understanding what gets rendered by the server versus the client to effectively fix hydration errors and maintain a seamless user experience.