Scientifically speaking, there are three meanings for a red moon: that there is a lunar eclipse, that the moon is near the horizon and the light is passing through a larger amount of atmosphere or that particles in the air (from events like a forest fire or a volcanic eruption) are partially obscuring the light. Any of these conditions cause the light from the moon to be partially scattered, eliminating the blue and green parts of the visible light spectrum so that only red light filters through.
A lunar eclipse is one of the more predictable causes of a red moon. Because the red moon is caused by a certain alignment of the sun, Earth and moon, astronomers are able to calculate when the next one occurs. In 2014, lunar eclipses take place on April 15 and October 8.