Refers to the portion of the near side
of the moon
which is lit
As the moon travels around the earth, it keeps the same side toward the earth at all times, and as a result, this side is called the "near side". The other side is referred to as the far side, or sometimes erroneously as the dark side of the moon. The lit portion of the moon changes over time, making a complete revolution in roughly the same time the moon makes a revolution around the earth, or about 29 days.
Since ancient times people have noticed this cycle in the appearance of the moon from earth, and the cycle is divided into several phases.
When the moon is closest to being directly between the sun and the earth, the entire near side of the moon becomes dark, and this phase is called the new moon. (If the moon passes directly between the sun and earth, it leaves a shadow on the earth, called a solar eclipse.)
When the earth is closest to being directly between the sun and moon, the entire near side is lit, and this phase is called a full moon. (Again, if earth passes directly between the sun and moon, it leaves a shadow on the moon, causing the full moon to become dim for a short while. This is called a lunar eclipse.)
There are various other names for the phases in between; see waning, waxing, quarter, gibbous, etc.