The iris is a flat and ring-shaped membrane behind the cornea of the eye that contains the pupil. The iris is the structure responsible for providing an individual with eye colour. More importantly, along with the pupil, the iris is responsible for controlling the amount of light reaching the back of the eye, or retina. Similar to the relationship between the diaphragm and aperture of a camera, the iris (i.e. diaphragm) adjusts the size of the pupil (i.e. aperture). It is the muscles contained within the iris that allow the pupil to become larger (open up or dilate) and smaller (close up or constrict).
Too much or too little light can affect or impede vision. The iris moves to shrink the pupil when there is too much light and widens when if there is not enough. This is an involuntary function, controlled by the brain.
Contrary to common belief, the colour of the iris rarely changes after six months of age. While the colour of an individual's eyes may appear to change, this is almost always due to changes in environmental lighting (e.g. shadows, reflections, etc.) or a misperception of colour based off adjacent colours. Nevertheless, one can temporarily change their eye colour with coloured contact lenses!