The URL to link to when the applet is clicked. This is the name of the frame you want any linked page to appear in. I'd suggest you put the class file in the same directory as the page on which it will appear. Some implementations of Java will not allow you to put the .class file in one subdirectory and the image file in another. If the try it might cause a security exception.
Some web browsers have trouble opening applets straight from a hard disk.
I have noticed this happen with Internet Explorer 3 and Netscape 4. This is an applet, it is intended for use within web pages. You can't open it directly. If you try, it won't harm your computer, but neither will it show you the applet. Class files are binary files, just like image files are.