React Strict Mode is a tool in React that highlights potential problems in a React application during development mode. It adds extra warnings to components inside `<React.StrictMode>` tags, but does not render any UI. This helps developers avoid legacy code and deprecated APIs by highlighting issues with lifecycle methods, string ref and context APIs, unexpected side effects, and deprecated APIs. To use Strict Mode, developers can wrap specific components or pages with the tag, or enable it globally in their Next.js application by setting `reactStrictMode` to true in their `next.config.js` file.