su

determiner

1. his, her, its, their

   • Su tío es de Texas.
     Her uncle is from Texas.

2. your (polite)

   • Aquí está su pedido.
     Here is your order.

Plural sus: sus hijos. Which owner is meant comes from the context.

Learn it in Unit 4: The family: More family: grandparents, uncles, cousins · A1