Basically Singers in the Kurzicks culture is just a position. A highly honored one that only a few get to be.
"Stone Singers" perform a ceremony which heal the stone of the land to maintain it. this is what the Arborstone mission is all about.
"Tree Singers" perform a ceremony which make people into Juggernauts. Which is what the The Eternal Grove mission is all about.
In the lore to me, they are nothing, but construction workers who use spells to build, structures or an army.