The Menu key is the one that brings up the window menu on windows, it normally has a picture of a menu on it. Same as clicking the icon on the left side of the title bar.
The Meta is found on some keyboards but on a pc you need to see if it's your shift+alt key or maybe your Super key.
If you have no Meta key run xev and try shift+alt
Group switching would apply to having multiple mappings to switch between. Normally for using different languages would be my first guess. This would not apply to a US keyboard.
Hyper is a non ascii control character like is used in the msdos editor by certain key sequences. May be mapped to right ctrl key, but I doubt if it's default.
The third level thing is about key modifiers. The first level or "level 0" is small case letters, the second being capitol letters. The third level I believe would also apply only on a non-US keyboard with groups. However I have never seen a non-US keyboard so I'm guessing.
