(Rails) What is "RJS"?
This Railscast gives a nice example of using RJS to add and remove form fields dynamically without hitting the server with an ajax call.
These RJS tips may also be helpful.
RJS is a template (similar to an html.erb file) that generates JavaScript which is executed in an eval block by the browser in response to an AJAX request. It is sometimes used (incorrectly?) to describe the JavaScript, Prototype, and Scriptaculous Helpers provided by Rails.
RJS was a "ruby-to-js" template system, RJS is not widely used anymore in the Rails ecosystem. SJR (Server-generated JavaScript Responses) is now prefered.
This DHH's post helped me a lot to understand : https://signalvnoise.com/posts/3697-server-generated-javascript-responses
2021 EDIT : now you should use Hotwire.dev