Sattva comes to prevail, O descendant of Bharata, by overpowering rajas and tamas. Rajas prevails by overpowering sattva and tamas, and in the same way tamas prevails by overpowering sattva and rajas.
Explanation
Krishna is explaining to Arjuna how the three qualities of nature (guṇa) work. Every person has all three: sattva (clarity, goodness), rajas (restless activity) and tamas (dullness, inertia). At any moment one of them overpowers the other two and shapes our mood and behavior, so the balance keeps shifting.