The 6 meridian theory refers to the 3 yang and 3 yin patterns known as Tai Yang, Yang Ming, Shao Yang, Tai Yin, Shao Yin and Jue Yin. This is also known as differentiation according to the 6 stages (or the 6 meridians) and could be translated as the 6 "levels" due to the progression of conditions from Tai Yang (most external) to Jue Yin (most internal). Using the theory of the 6 stages you can see a progression of conditions from external to internal. By treating the correct "stage" you can avoid, for example, either driving the condition in deeper or blocking it's way "out" of the body (i.e. tonifying when you need to disperse an external pathogen such as a cold).
The times, etc. are not entirely part of this theory per se but of Chinese Medicine generally. All of these relationships are described on my acupuncture points diagram (shown below, taken from the acupuncture points database). Many people miss the information on the graphic because they are not aware of all of these theories. The graphic shows not only the obvious meridian names but also these 6 stage meridian pairings (tai yang, shao yang, etc.), the meridian activity times based on the Chinese Clock (when the meridians are most active and conversely when they are least active), and the entry exit points (note - not always the first and last points on the meridians, see the entry exit points theory section for more information).
In the above graphic you see that the heart and kidney meridians form the shao yin and the small intestine and bladder meridians form the tai yang. As an example of meridian flow you see that the qi exists at KD 22 and enters the pericardium meridian at PC 1 and then exist PC 8 and enters the Triple heater meridian at TH 1, etc. You also see that the pericardium is the most active between 7-9pm and the liver is most active between 1-3am (see my article on circadian rhythms for more information).
The 6 stages or levels then are as follows:
More symptomatic details, for reference sake: