Swf files are the flash applications that you see on some websites. Even if developers usually know that the files can be decompressed and avoid putting sensitive stuff inside, there are still leaks.
To get the path directly for a swf file on a website, you can sometimes just spot it in the url directly. Other times you can use a Google search for a specific domain (in case it’s public) by entering “site:domain.com filetype:swf”. Other times the swf are little bit hidden and you need a tool like Fiddler2 to find the links.
Once you get the swf file, download it to your computer and decompress it. The decompress can be differently by different tool, but I recommend https://sites.google.com/site/as3extractor/ or http://www.showmycode.com/ShareThis