What is Prakriti?
While Purusha, the Self, or Pure Being exists as the infinitely silent, formless witness, Prakriti is the infinitely dynamic, ever changing universe, in all of its forms and processes. Prakriti could also be called "Pure Doing" as the nature of Prakriti is that is is always in action.
Everything around you, all matter, energy, every thought and idea is an expression of Prakriti.
The relationship between Purusha and Prakriti gives the experience of existence.
Like Yin and Yang in Taoist philosophy, Purusha and Prakriti rely on each other. Without Prakriti, Purusha has no vehicle of expression. Likewise, Prakriti is has no value without Purusha experiencing it.
Purusha experiencing itself through the lens of Praktiti gives us the experience of existing. This relationship between Purusha and Prakriti, between subject and Object is called Lila, or the divine "play". The whole universe is just the play between Purusha and Prakriti.
The whole experience of Purusha, Prakriti and Lila is called Brahma, the creator.
Lila is the creator or Brahma knowing itself in all possible forms again and again.
"Taking Recourse to my own eternal nature, I create again and again."
-Bhagavad Gita 9.8