A Full Moon happens when the Moon is in opposition to the Sun (180°). It is the half-way point of the 29-day lunar cycle.
It is usually a time of high energy, when emotions are fully open. For 1-3 days before and after a Full Moon, we may be more reactive, more sensitive, more easily overwhelmed or triggered, or generally experience feelings more deeply.
This is a time when the themes that began at the New Moon (about 14 days before) come to fruition or full emergence. In the 2 weeks that follow the New Moon, we see the Moon's light expand (from right to left) until it shows a bright circular disk, which is the Full Moon. After the Full Moon, the Moon's light shrinks (from right to left) until it is no longer visible because it is too close to the Sun.
A Full Moon can also be a Lunar Eclipse. This typically happens twice per year, about 5-6 months apart, and occurs when the Sun and Moon are both opposite and contra-parallel, and within about 18° of the Lunar Nodes axis.
Related articles: Sun and Moon ~ The Lunar Cycles : Tripping the Lights Fantastic and Blue Moon Lunacy.