The word Kala usually refers to the extent by which the moon waxes (or wanes) in one day. So by this understanding, the Moon possesses 16 different Kalas (different phases of the Moon). You can observe this from one Amavasya(No-moon day) to the next Purnima (Full moon day). So if you count the number of days from one Amavasya to the next Purnima, it turns out to be 16. And the transition from No-Moon to the Full Moon also symbolizes moving from Zero (nothingness) to Fullness.
On the no-moon day you don’t see the moon at all, and on the full moon day you see the complete moon. This does not mean that the Moon is not there on the day of Amavasya. No, it is still there. So by this understanding, these are called the Solah Kalas. On the day of solar eclipse, for example, the Moon is there but it cannot be seen, that’s all.
So the word Kala is a unit of measure of one’s power, or by what degree one has blossomed or progressed. So, someone may be like a half-moon, someone else may have reached to three quarters of the moon and so on. This was a means of measurement in the ancient days. In those days, it was said that the life or Prana in a stone is equal to one Kala, the water element is said to have two Kalas. Fire has three Kalas, Vayu (Air element) has four, and the Space or Akash has five. Plants and trees are said to have six Kalas. Animals have seven Kalas. Human beings have eight Kalas. If it is an extraordinary human being then he is said to have nine Kalas. It is said that Lord Parashurama (one of the ten avatars of Lord Vishnu) had nine Kalas. They say Lord Rama has 12 Kalas. Lord Hanuman is greater than Him in this regard and is said to have 14 Kalas. That is why he could help Lord Rama. Only a person who is stronger than us can help us.
The Devi (Mother Divine) is called Shodashakala (The One with all 16 Divine qualities or attributes). It means that she possess all divine and noble qualities, and lacks nothing. Lord Krishna too is said to have all 16 Kalas, which is why He is called as the Solah Kala sampurna or the Purna Avataar. To have all 16 Kalas means to possess all the 16 qualities or signs attributed to the total blossoming and manifestation of Divinity.