The sunlit fraction of the Moon never changes ... it's always 50% .
What changes is how much of that 50% we can see from Earth.
The Moon is not doing anything special while that happens.
The moon wanes.
It is the waning phase.
When the moon is bright, the sunlit part of the moon that we can see is getting larger. When the moon is dark the sunlit part of the moon that we see is getting smaller.
It is the waxing phase of the moon.
This indicates that the moon is waxing, meaning it is transitioning from a new moon to a full moon. As the days progress, more of the moon's sunlit side becomes visible from Earth.
A waxing moon has the light on the right, and is heading towards a full moon (getting bigger).A waning moon has the light on the left and is getting smaller.
Waxing. The moon is waxing as it goes from new (unlit) to full (100% lit).
The gibbous moon appears to grow fatter each night until we see the full sunlit face of the Moon. We call this phase the full moon. It rises almost exactly as the Sun sets and sets just as the Sun rises the next day. The Moon has now completed one half of the lunar month while orbiting earth. Hope this helps!
The apparent change in the sunlit portion of the moon through the month is called waxing (getting larger, from crescent through full) or waning (getting smaller, from full down to crescent).
It is the waxing phase.
waxing
After the full moon, we see less and less of the sunlit side.