The `retry()` operator is used in RxJS to resubscribe to an observable if an error occurs. The operator will automatically resubscribe to the source observable, potentially with a delay or other customization options, and continue emitting values to its subscribers. The `retry()` operator can be useful in scenarios where the observable may fail due to intermittent network errors or other issues, allowing the application to recover from these errors and continue operating. Here's an example of how to use the `retry()` operator: 

import { of } from 'rxjs';
import { map, mergeMap, retry } from 'rxjs/operators';

const source$ = of('http://my-api.com/data');

const data$ = source$.pipe(
  mergeMap(url => fetch(url)), // Assume fetch() returns a promise with the data
  map(response => response.json()),
  retry(3) // Retry up to 3 times if an error occurs
);

data$.subscribe(
  data => console.log(data),
  err => console.error(err),
  () => console.log('Complete')
);
In this example, the `source$` observable emits a single value - a URL that points to an API endpoint. The `mergeMap` operator is used to call the `fetch()` function with the URL and return a promise that resolves with the response data. The `map` operator is then used to parse the response data as JSON. The `retry()` operator is used after the `map()` operator to specify that the observable should be retried up to 3 times if an error occurs. If an error occurs during the execution of the observable, RxJS will automatically resubscribe to the source observable up to 3 times, allowing the application to potentially recover from network errors. It's worth noting that the `retry()` operator can be customized with additional options, such as a delay between retries or a predicate function that determines which errors should trigger a retry. These options can be useful for handling more complex scenarios.