(Rails) What is "RJS"?

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

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.