Before you can secure a React app, you need to understand two key concepts: authentication and authorization. Authentication is the process of verifying who a user is. It answers the question: Are you really who you say you are? Authorization, on the other hand, determines what an authenticated user is allowed to do. It answers the question: Now that we know who you are, what are you permitted to access or modify? Both are essential for keeping web applications secure: authentication ensures users are who they claim to be, while authorization ensures users can only access resources they are allowed to use.

Think about logging in to an online banking site. When you enter your username and password, the site checks your credentials — this is authentication. Once you are logged in, you might be able to view your account balance but not access another user's account — this is authorization. In a React app, you typically use authentication to identify users and authorization to control access to different parts of your application, such as admin dashboards or user settings.

Imagine another scenario: you use a social media app where you sign in with your email. The app checks your credentials (authentication). After signing in, you can only edit your own posts, not those of others (authorization). These real-world examples show how authentication and authorization often work together but serve different purposes.