Ghayb signifies the verities which are hidden from man's senses and which are beyond the scope of man's ordinary observation and experience, for example the existence and attributes of God, the angels. the process of revelation, Paradise, Hell and so on.
According to verse 2:3 of the Holy Quran, belief in Ghayb is part of Faith. 'Belief in the ghaib' means having faith in such matters, based on an absolute confidence in the Messengers of God and despite the fact that it is impossible to experience them.
According to this verse, Qur'anic guidance can prove helpful only to those prepared to affirm the truths of the suprasensory realm. People who make their belief in these questions conditional upon sensory perception of the object of belief, and who are not prepared even to consider the possibility of the existence of things that cannot be weighed or measured, cannot profit from the The Holy Quran.