To do this, add the following line at the end of that file. You need to provide the name of the Virtual Host as well as the root/main folder of your website. This step prepares the Web server on your computer for handling symfony.local requests. On MAMP, you can find this file here: Applications/MAMP/conf/apache/extra/nf. On XAMP, you can find this file here: C:\xampp\apache\conf\extra\nf.
You first need to set up a virtual host in your apache nf file. Let's say you're a big Symfony2 fan and you would like to access your symfony website at from 4 different computers (the main one hosting your website, as well as a Mac, a Windows and a Linux distro connected (wireless or not) to the main computer. We'll take a general case with Apache here.