The Tithi or lunar day before the full moon is called Purnima and indicates the time when the Moon is at its fullest. After the exact time of the full moon, the light will decrease so Purnima is when the moon shines the brightest.
As I wrote on the last new moon, Guru Purnima is the day to show gratitude to all the teachers who generously taught us invaluable knowledge.
We also have an inner guru within us. But our mind is full of desires and wishes and it is difficult to listen to the voice of this inner wisdom. We have to aim to clear this clouded state of mind by setting clear intentions, doing Yoga and meditating, so that our intuition can start listening clearly to the voice of this inner wisdom. This wisdom is our inner Guru. It is a time to be grateful to this Guru as well. Om Shanti.