Definition - What does Nirguna mean?
Nirguna is a Sanskrit term that may be translated as “without distinction." It's an important term in the philosophy of Vedanta, which shares its roots with yoga, because it's often used to raise the question of whether Brahman is with or without qualities (distinction).
Some schools believe that Brahman is nirguna in all respects because the being is beyond all polarity and cannot be characterized using standard measures. Others, however, believe that Brahman possesses perfect characteristics and qualities, and, as such, is only without the negative or imperfect qualities.
Yogapedia explains Nirguna
The text that particularly raises the question of whether Brahman is nirguna or not is the "Brhadaranyaka Upanishad," which defines Brahman as neti-neti, or “not this, not that.”
An alternative perspective of nirguna and its opposite, saguna, is that they are two aspects of God. Nirguna is the eternal omnipresent Divine Consciousness. Saguna, on the other hand, is the manifestation of God in form. It is said that when God appears in form, although his form appears to be limited, his presence is unlimited.
Some worshippers say that God in the form of nirguna Brahman is too subtle to comprehend easily. Instead, they might use a saguna Brahman, or a way of perceiving God with recognizable qualities,in order to help them connect with God on a simpler level.