What Do the Moon Phases Mean?
The Moon makes no light of its own; what we see is the sunlit half turned towards us. As the angle between Sun, Earth and Moon shifts, that lit portion grows and shrinks. One full round takes about 29.5 days and is divided into eight phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, waning crescent. The cycle does not sit inside a calendar month, so the full moon slides by a few days each time and some months take two of them.
Traditional reading follows that visual rhythm closely. While the light is increasing, from new moon to full, the stretch counts as a time for beginning things, gathering and turning outward. While it is decreasing, from full moon back to new, it is tied to letting go, cutting back and turning inward. The quarters are the points of tension between those halves: the first quarter reads as decision, the last as review.