When a group of people are singing or playing music together, there are usually different parts that need to be played or sung. Each part has its own special melody, or tune, that is different from the others. The most important part is called the hauptstimme, which is German for "main voice".
Think of it like a cake. The hauptstimme is the biggest and most important part of the cake, like the sponge or the frosting. The other parts, like the sprinkles or the fruit, are still important and add flavor, but they don't have the same importance as the main part of the cake.
When musicians are playing together, they need to make sure that they are all playing the hauptstimme correctly, because it sets the foundation for the rest of the music. Just like in a cake, if the main part isn't done right, the whole thing can fall apart.
So, the hauptstimme is like the most important part of a cake or a musical piece, and it sets the tone for the rest of the performance.