What is the iris in the human eye and what does it do?

The iris regulates the amount of light that passes onto the retina of the eye, by reducing its visual area in a bright light. It accomplishes that by regulating the size of the pupil by contracting and expanding depending on amount of light.

The iris is a membrane in the eye, responsible for controlling the diameter and size of the pupil and the amount of light reaching the retina. "Eye color" is the color of the iris, which can be one of several, depending on genetics.