Suppose your boss has asked you to make some changes to the website what would you do? One would first have to make changes to a particular file or sometimes multiple files. Now how would you find out which file to edit? Most of the popular browsers have luckily come out with developer tools.
The developer tools allows you to literally inspect an element of a Web page. It would show you the css and html code. And it will also give you the file name.
One could also make live edits to a page, to see how it works. You could test the code before making the actual change in the backend.
Firefox and Chrome are the most sought after developer tools right now. In order to activate the developer tools, one would have to right click at the point of interest and select Inspect element.