Lunar phase
A lunar phase is one of the shapes the Moon takes across a roughly 29.5-day cycle, as the angle between the Sun, Earth and Moon shifts. The visible lit portion moves from the new moon (a dark disc) to the full moon (a fully lit disc) and back toward darkness. Tradition reads this cycle as a kind of breathing, from first impulse to ripeness and then release.
What you see in the sky
The Moon makes no light of its own: you see the slice of its face that the Sun lights and that happens to face you. When it sits near the Sun in the sky, its dark side looks your way. When it sits opposite the Sun, you see it full. In between, the crescent swells and then thins, and the same cycle repeats every month.
What the tradition invites you to do
Many cultures tuned their rhythms to this cycle: when to sow, harvest, rest. You can treat it as an inner calendar, where the new moon welcomes your intentions and the full moon shows what has come to ripeness. Think of it as an invitation to follow a rhythm, not a rule, and feel free to live it your own way.