The three gunas are, in the Samkhya philosophy of India and in the Bhagavad Gita, the three fundamental qualities or strands of which prakriti, primordial material nature, is woven; everything manifest is a mixture of the three in shifting proportion. Sattva is the guna of purity, harmony, clarity and balance, the quality of light, intelligence, calm and goodness, the strand that reveals and illuminates.
In the Samkhya scheme the three gunas are not moral categories imposed from outside but the actual constituents of nature, and sattva is the finest and most transparent of them, associated with knowledge, serenity and the upward pull toward the spiritual. The Bhagavad Gita, especially in its fourteenth, seventeenth and eighteenth chapters, classifies food, action, knowledge, worship and temperament by their predominant guna, and the cultivation of sattva over the restless rajas and the dull tamas is presented as a stage on the path, though the ultimate aim is to pass beyond all three. The doctrine is ancient and central to Hindu thought.
Sources Classical Samkhya: An Interpretation of Its History and MeaningGerald James Larson with Srimad-Bhagavad-Gita, English translation and commentarySwami Swarupananda