these terms refer to the phases of the moon. as the moon orbits the earth, different parts of it are illuminated by the sun. since the moon always presents the same face to earth, starting from the full moon, we see that the phase first wanes (decreases) until we have a new moon, then waxes (increases) until we have a full moon again. the whole cycle takes about a month (in fact that was the definition of the month, in some older 'lunar' calendars). 'gibbous' refers to the three quarters full phase, either waxing or waning.
