For an explanation about normalization and "Replay Gain" read here: Equalizer and Replay Gain

Normalization (Replay Gain) Settings

Always use Replay Gain per track (not recommended).
Replay Gain is calculated per track and per album. For albums the album value is used because then for the whole album the same sound level is used. For track collections the track value is used. With this option, you can choose to use track values always.  

Normalization (Replay Gain) Pre-amplify  

Replay Gain values normally makes music more quiet. To compensate, you can enter a value for pre-amplification. A value of 6dB is most of the time ok for pop music, a value of 0dB is better for classical music. Putting the value to high can cause clipping, which means distortion.

Normalization (Replay Gain) Calculation

Automatically calculate Replay Gain for newly added tracks.
With this option the AlbumPlayer will automatically calculate Replay Gain values for tracks which are added to the database. Note that calculation is extremely time consuming when adding large collections.