Rational self-interest
Each person's own life is their highest value, pursued by reason and without sacrificing others.
Rand's ethics holds that a person should act for their own long-term flourishing, judged by reason, neither sacrificing themselves to others nor others to themselves. She called this "selfishness" on purpose, to reclaim the word.
The common misreading
It is not licence to exploit others. Rand rejected gaining at others' expense as irrational and destructive of one's own interests.
Read it in Rand’s words
The Virtue of Selfishness ("The Objectivist Ethics")