An auto iris adjusts to the ever-changing light under outdoor conditions like the human eye. When the sun shines, the iris closes automatically so that less light to the image processor falls. Is it, for example darker through a cloud, it opens the iris to allow more light to come in.
There are in addition to the auto-iris still: Fixed Iris, Manual Iris and P-Iris. A full explanation can be found at AXIS Communications.
Only Auto-Iris allows camera using a in outdoor areas. Without this feature, a camera is not suitable for all light conditions and not for perfect outdoor use.