What is javascript rendering?

I'd like to get a good grasp of what javascript rendering is. I know a few things but I can't stitch them together. As far as I understand, when a URL or a request is sent, HTML and CSS files form the Document Object Modifier which then javascript renders to produce the output web page.

Also, somewhere I read that javascript code should be rendered first for google to get a sense of what the content is about. I hope I got that right. If so, at what stage does this happen?