Qeros are made and used in pairs, and the pairing carries the meaning. Two matching vessels are filled with aqha, the fermented maize beer generally called chicha, and two people drink at the same time. The object does not simply hold a drink; it enacts a relationship between two parties, each of whom gives and each of whom receives, at the same moment and in the same measure.
That structure of matched obligation runs through Andean social life, and the qero is its most portable expression. Toasts were exchanged between individuals, between a lord and a subordinate, between kin groups settling something, and between people and what is not human: the sun, the ancestors, and the huacas, the powers of particular places. An agreement made without the shared drink was not properly made.
The form is a flared beaker, wider at the lip than at the base, sized to be held in one hand. Inca examples are wooden, carved with geometric registers, and vessels in silver and gold and in pottery exist alongside them. Nothing about the shape is complicated, and its persistence across centuries has more to do with what it is for than with how it looks.
Wood is the usual material and the choice is practical as well as customary: a wooden vessel does not chill or heat the aqha, survives being handled and set down repeatedly, and can be carried in numbers.