What does registerServiceWorker do in React JS?
The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that's what you should know about service worker's for now. It's all about adding offline capabilities to your site.
React creates a service worker for you without your configuration by default. To learn more;
The service worker is to use the website data offline in case there is no network, it's like making PWA. Refer to this link PWA to know more about implementation and how to use registerServiceWorker().