Mallets is a sample-based synth emulating tuneful things to bang on :)
There are three different categories, and the settings are accordingly different depending on the sound you are selecting.
Percussion instruments: Marimba, Vibraphone, Agogo, Wood1, Reso, Wood2, Beats, Two Fixed and Clumps
Bars: Uniform Bar, Tuned Bar, Glass and Tibetan Bowl
You can change the basic sound by clicking on the name in the box. Left-click to step forward, right-click to go backward. You can also click on the arrow and will get a drop-down menu with all included sounds.