What is output buffering in node?

I'm programming in node.js using the express framework. The default template engine in express is Jade. In the Jade specification there is a concept mentioned called output buffering. What exactly is happening when I use this?

This is referring to the fact that executing JavaScript code with an = in Jade adds the result of that JavaScript to the buffer--that is, the text that is returned after the Jade template is parsed. - also executes JavaScript code, but does not add its output to the buffer.