It's a monitor artifact that is all too common. Adjusting it for photography by reducing the brightness from the excessive factory default and calibrating it can worsen it or make it visible.
CRTs are analog devices that can smoothly vary the intensity of the electron beam. But an LCD monitor can "twist" the liquid crystals to admit the backlight only in discrete steps. On all but the most expensive monitors intended for graphics professionals, there are only 256 steps per color (some only allow 12

. Calibration works by adjusting a look-up table in the video card to alter some of the levels sufficient to produce a standard brightness curve for each color. That can produce (or increase) visible banding in gradients.
A gradient will reveal banding that might not be visible in real-world images. So it may not be a problem, or it might be so limited that you can live with it. If banding is excessive, or visible in images without computer-generated gradients, you may need a high-end monitor that uses 10-bit color rather than 8-bit.