what is Redirect URI?

private static final String CLIENT_ID = "a00f335fbd1849bc8841cf3ab3f31b00";

private static final String TAG = MainActivity.class.getSimpleName();

private static final String REDIRECT_URI = "SpotifyTestApp://callback";

private static final int REQUEST_CODE = 1337;

private Player mPlayer;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    AuthenticationRequest.Builder builder = new AuthenticationRequest.Builder(CLIENT_ID,
            AuthenticationResponse.Type.TOKEN,
            REDIRECT_URI);
    builder.setScopes(new String[]{"user-read-private", "streaming"});
    AuthenticationRequest request = builder.build();
    Log.e("Spotify aa " , "the request is sent");

    AuthenticationClient.openLoginInBrowser(this, request);

}

@Override
protected void onNewIntent(Intent intent) {
    super.onNewIntent(intent);

    Uri data  = intent.getData();
    Log.e(TAG, "data");
    if (data != null) {
        AuthenticationResponse response = AuthenticationResponse.fromUri(data);

        switch (response.getType()) {
            // Response was successful and contains auth token
            case TOKEN:
                // Handle successful response

                Log.e(TAG, "the response is success");
                break;

            // Auth flow returned an error
            case ERROR:
                Log.e(TAG, "the response is fail");
                // Handle error response
                break;

            // Most likely auth flow was cancelled
            default:
                // Handle other cases
        }
    }
}



@Override
protected void onDestroy() {
    Spotify.destroyPlayer(this);
    super.onDestroy();
}

@Override
public void onLoggedIn() {
    Log.d("MainActivity", "User logged in");
}

@Override
public void onLoggedOut() {
    Log.d("MainActivity", "User logged out");
}

@Override
public void onLoginFailed(Throwable error) {
    Log.d("MainActivity", "Login failed");
}

@Override
public void onTemporaryError() {
    Log.d("MainActivity", "Temporary error occurred");
}

@Override
public void onConnectionMessage(String message) {
    Log.d("MainActivity", "Received connection message: " + message);
}

@Override
public void onPlaybackEvent(EventType eventType, PlayerState playerState) {
    Log.d("MainActivity", "Playback event received: " + eventType.name());
    switch (eventType) {
        // Handle event type as necessary
        default:
            break;
    }
}

@Override
public void onPlaybackError(ErrorType errorType, String errorDetails) {
    Log.d("MainActivity", "Playback error received: " + errorType.name());
    switch (errorType) {
        // Handle error type as necessary
        default:
            break;
    }
}

<uses-permission android:name="android.permission.INTERNET"/>

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
        <intent-filter>
            <action android:name="android.intent.action.VIEW"/>
            <category android:name="android.intent.category.BROWSABLE"/>
            <category android:name="android.intent.category.DEFAULT"/>
            <data android:scheme="SpotifyTestApp" android:host="callback"/>
        </intent-filter>

    </activity>

</application>