The iris is a flat ringlike membrane behind the cornea of the eye with an adjustable circular opening in the center called a pupil. It is where the eye gets its color.
Together with the pupil, the iris is responsible for regulating the amount of light that gets into the eye. Too much or too little light can hamper vision. The muscular iris moves to shrink the pupil if there is too much light and widen it if there is not enough. This is an involuntary function controlled by the brain.
The iris is made almost entirely of connective tissue and smooth muscle fibers.
Despite a common belief, the changing color of the iris rarely happens. While the color of an eye may appear to change, this is typically due to lighting changes or perception based off nearby colors.