Nibbaana is the summum bonum of Buddhist practice, to be achieved only by following the Noble Eightfold Path.
For most of us the journey along the Path will be long and arduous, but there are sign-posts on the way that will indicate we are going in the right direction.
We will recognize these sign-posts when the fetters that bind us are broken in succession.
When the first three fetters — personality view, doubt, and clinging to mere rules and rituals — are broken one becomes a “stream enterer” (sotaapanna), one who has entered the stream to nibbaana.
The fetters, once broken, will never bind such a person again. This is the truth he knows without uncertainty. The stream-enterer will not be reborn in the four lower planes of existence. He will take rebirth seven times at the most, either in the human or heavenly planes.