Define a callback route
To sign users in, your application redirects the browser to an Okta-hosted sign-in page. Okta then redirects back to your application with information about the user. You can learn more about how this works on Okta-hosted flows.
The first thing that you need to define is how Okta calls your app after a user is authenticated. This is called a callback route or redirect URI.
The callback route is not seen by the user, and it's not the user's final destination. It's just one step in the authentication redirect flow.
Note: It's important that the full URL of your callback route represents a real URL (in other words, it should serve your single-page app even after a "hard" browser reload). Most router components do this by default.