Moon phase describes how much of the moon's visible surface is illuminated on a given date, ranging from new moon (0%) to full moon (100%).
Moon phase drives two opposite photography goals, which is why it belongs on every night-shoot plan. A bright, near-full moon acts as a soft natural floodlight, useful for showing landscape detail in a nightscape or as a subject in its own right at moonrise. That same brightness lifts the whole sky's light level enough to bury the Milky Way and the fainter stars, so anyone shooting the galactic core wants the days around a new moon instead.
The illuminated fraction climbs fast near the middle of the cycle, so a moon at 40 percent is far less of a problem than one at 70. Timing counts as well as phase: a bright moon that has already set by 1 am leaves the rest of the night dark, so a late-rising or early-setting gibbous moon can still allow deep-sky work in the hours it is down.
A photographer plans a two-night trip hoping to shoot both a moonrise over a ruined abbey and the Milky Way over the same building. The moonrise shot wants a full or nearly full moon rising near sunset, low and large behind the abbey. The Milky Way shot wants that moon gone. These cannot share a night, because a full moon is above the horizon all night by definition.
The fix is to run the moonrise shot on the evening of the full moon and the Milky Way shot about a week later, once the moon has thinned to a crescent that sets in early evening and leaves the core hours dark. Checking an illumination percentage rather than the named phase avoids surprises, since a moon called a 'quarter' is still lighting the sky at 50 percent.
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