The A index is the level of geomagnetic activity. Geomagnetic activity includes anything that upsets the magnetic field that surrounds the earth. The A index can range from 0 up to 40. A lower number indicates that the geomagnetic field is less disturbed. Quite bands, therefore, correspond to low A indexes.
Quiet bands allow weaker signals to come through more clearly, as there is less noise to interfere.
Another interesting phenomenon to note is that the sun rotates once every 28 days or so. This is interesting because you can predict that the conditions may be similar to the conditions right now. With this basic knowledge, you will now be able to interpret the WWV transmissions in order to maximize your enjoyment from the hobby.