What are Minecraft class files?

I'm trying to install Dr. Zhark's Mo'creatures mod. I went to the Installing Mods website on minecraft.gamepedia.com, and I'm on step 9:

"Open the class files from the mod with an archiver such as WinRAR or 7-Zip and copy-paste the files into 1.6.4_Modded.jar. Do not close the archiver yet."

But I have no idea what class files are or where to find them.

Katoptris

The ones with the.class extension. Edit preferences, folder options (something like that) and turn on extensions visibility and you will find your class files.

Efri