It is concave due to the law of increasing opportu
It is concave due to the law of increasing opportunity costs.  Any point outside of it is unattainable, any point inside it is attainable but inefficient, and points on it are attainable and the most efficient.  It can move to the right if resources increase in quantity or quality or technology advances, but it normally assumes full employment, fixed resources, fixed technology, and a two-product economy.  FTP, name this curve which relates the maximum outputs of two goods that use similar resources.
It is concave due to the law of increasing opportunity costs.  
production possibilities curve or production possibility curve