Since you have come to this file, I'm assuming you want to know about .swf files. .swf files are files that use a Flash player. Since some of these Scratch mods are Flash-based, that is the reason they use the .swf file extension. Since Chrome is dropping support for Flash, you're going to need an old version of Chrome. Here are some download links:
Windows 32-bit: https://www.slimjet.com/chrome/download-chrome.php?file=win%2Fchrome32_48.0.2564.97.exe
Windows 64-bit: https://www.slimjet.com/chrome/download-chrome.php?file=win%2Fchrome64_48.0.2564.109.exe
Ubuntu/Debian 32-bit: https://www.slimjet.com/chrome/download-chrome.php?file=lnx%2Fchrome32_48.0.2564.109.deb
Ubuntu/Debian 64-bit: https://www.slimjet.com/chrome/download-chrome.php?file=lnx%2Fchrome64_48.0.2564.109.deb
MacOS: https://www.slimjet.com/chrome/download-chrome.php?file=mac%2Fchrome_48.0.2564.109.dmg
You'll also need a Flash player, which can be found online here: http://flashplayer.fullstacks.net/