The Quality of Purity
Sattva ('being,' 'goodness,' 'clarity') is the first of the three guṇas (fundamental qualities of Prakṛti) in both Sānkhya and Vishishtadvaita cosmology. The three guṇas — sattva, rajas, and tamas — are the constitutive qualities of all material existence. Sattva is the quality of clarity, light, purity, knowledge, and harmony.
Effects of Sattva
When sattva predominates in a person's antaḥkaraṇa (inner instrument/mind), they experience clarity of thought, ethical uprightness, love for learning, equanimity in pleasure and pain, and natural inclination toward devotion and liberation. Sattvic food (light, nourishing, naturally grown), sattvic associations (satsaṅga with devotees), and sattvic activity (study, meditation, worship) increase sattva.
Śuddhasattva
Beyond ordinary sattva is śuddhasattva — 'pure sattva' unmixed with rajas or tamas. In Sri Vaishnava theology, śuddhasattva is the very stuff of the divine realm: Vaikuṇṭha and the Lord's divine body (divyamaṅgalavigraha) consist of śuddhasattva, which is the Lord's own nature made material. This is distinct from the triguṇātmaka (three-guṇa-comprising) sattva of the ordinary material world.
Transcending the Guṇas
Ultimate liberation involves transcending all three guṇas, including sattva. The Bhagavad Gītā teaches going beyond the guṇa-triad (triguṇātīta). But in the path toward liberation, increasing sattva is the necessary step — and for Sri Vaishnavas, association with bhāgavatas and immersion in Divya Prabandham is the most powerful means of cultivating sattva.