When the moon appears orange or yellow, it simply means that the observer is looking at it through more layers of atmosphere. Similar to the way in which the sun will look orange or red on the horizon, when the moon is low in the sky, the light from it will have to pass through more of the blue-absorbing atmosphere to reach the viewer's eye. By this time, only yellow, orange and red light will remain unabsorbed.
A yellow moon is commonly called a Harvest Moon. However, the phrase is more a reference to its earlier rising time during fall, which often provides more light after sunset during harvest season. The optical illusion of an increased moon size or proximity that often accompanies this yellow color is called the moon illusion. It is not the same as a supermoon, which is when the moon is actually closer to the planet.