What is the meaning of "Level" and "Leveling" amplifier parameters ?

The "Level" parameter is the target loudness value that the amplifier will try to reach across the tracks. Note it has nothing to do with the device volume, it affects only the way the Amplifier works. Of course, if you lower this parameter, the output sound will be less loud for all tracks. If you push it to the limit, you may not hear a big difference as the amplifier limits its action to prevent clipping.

The "Leveling" parameter affects how amplifier keeps original track dynamics. If you set it to "Light", the quieter parts of the tracks will be kept much lower than the louder parts, if you set it to "Maximum" every part of the track will have nearly the same volume (which can degrade somewhat music interpretation, but will help in very noisy environments).