You can also read and listen to this episode on our blog: https://islamic-audio-book.com/14-calculating-the-lunar-year/
The Moon is a means of measurement for people to calculate the year. The positions of the Moon as it revolves in its orbit change, making it possible to see the Moon in different forms at different times.
In former times, a month was calculated by the time between two full moons, but in the Gregorian calendar, a year is the time it takes the Earth to orbit the Sun. The Qur'an contains information about this difference in calendars.