First we need to understand the four levels of enlightenment or stages of deliverance according to the Buddha’s teaching: Stream-entry, Once-returner, Non-returner and Arahant. A Stream-entrant has almost destroyed three lower fetters. A Once-returner has almost destroyed the five lower fetters. A Non-returner has destroyed the five lower fetters completely, and partially destroyed the five higher fetters as well. An Arhant has completely destroyed all the ten fetters.
The lower fetters are identity view, doubt, the distorted grasp of rules and vows, sensual desire, ill will. The higher fetters are lust for form, lust for the formless, conceit, restlessness and ignorance.
And what about the qualifications of the Stream-entrant?
“What are the four factors of stream-entry that he possesses?
1—“Here, householder, the noble disciple possesses confirmed confidence in the Buddha thus: ‘The Blessed One is an arahant, perfectly enlightened, accomplished in true knowledge and conduct, fortunate, knower of the world, unsurpassed leader of persons to be tamed, teacher of devas and humans, the Enlightened One, the Blessed One.’
2—“He possesses confirmed confidence in the Dhamma thus: ‘The Dhamma is well expounded by the Blessed One, directly visible, immediate, inviting one to come and see, applicable, to be personally experienced by the wise.’
3—“He possesses confirmed confidence in the Saṅgha thus: ‘The Saṅgha of the Blessed One’s disciples is practicing the good way, practicing the straight way, practicing the true way, practicing the proper way; that is, the four pairs of persons, the eight types of individuals—this Saṅgha of the Blessed One’s disciples is worthy of gifts, worthy of hospitality, worthy of offerings, worthy of reverential salutation, the unsurpassed field of merit for the world.’
4—“He possesses the virtues dear to the noble ones—unbroken, untorn, unblemished, unmottled, freeing, praised by the wise, ungrasped, leading to concentration.” — Pañcaverabhaya Sutta (SN 12.41)