The purpose of a parable is to make the meaning of an abstract idea more clear and explicit with the help of a material thing in order to assist the mind in catching the idea through consideration. The more man's wisdom is dependent on his base understanding, the less vigorous strength he has to conceive the spirit and reality of things when his imagination interferes.
That is why that in the Divine Book and in the words of the wise and men of letters, the essence of meanings are stated in the form of parables to make complicated concepts plain, hence through a logical thing find a way to the philosophical.

A 'gnat' is a small insect which is perceptible by the naked eye. Gnat is mentioned in this verse, perhaps, to show that striking a similitude between small things or beyond it is not inappropriate for the Glorious Dignity of the Essence of Allah, but to confirm that the proportion of His Sovereignty is the same over big or small, high or low and, in general, all are encompassed by His Power and are benefited by His endless Mercy and Grace.

Furthermore, if we observe carefully, we will see that all the creatures, whether complex or simple, like a chain, are linked to one another. They are so interrelated and perform their duties with such surprising harmony and regularity that it is as if they have formed a single unit to take part in a general service.

The term 'gnat', in this verse, may refer to the greatness and fairness that is used in the creation of a gnat, this very small creature.

Concerning this meaning, there is a tradition from Imam al-Sadiq (as) who said:

"Verily, Allah (s.w.t.) has set forth a parable of a gnat because He has created 'gnat', with that small size it has, consisting of all that He has created in {an elephant' with that bigness it possesses, and He has added two other limbs (for gnat) in order to stimulate (the attention of) the believers to notice the delicacy and frailness of His creation and His wonderful production. " 10

In another occurrence in the Qur'an, He (s.w.t.) has said:

"Do they not look at camels, how they are created?" (Surah Al-Qashiyah. No. 88. verse 17).
Upon the importance of the intriguing creation of living creatures, Amir-ul-Mu'mineen Ali (as) in one of his sermons has said: "…How could it be? Even if all the animals of the earth whether birds or beasts, stabled cattle or pasturing ones, of different origins and species, dull people and sagacious men –all jointly try to create (even) 'a gnat' they are not able to bring it into being and do not understand what is the way to its creation.

Their wits are bewildered and wandering. Their powers fall short and fail, and they return disappointed and tired, knowing that they are defeated and admitting their inability to produce it, also realizing that they are too weak (even) to destroy it…" 11

The verse continues saying:

“…As for those who have Faith, they know that it is the truth from their Lord…"
Those who have believed in Islam and accepted the Word of Allah know that these miraculous statements are issued from the Origin of revelation. They know quite well that these parables are vivid and appropriate and are revealed from their Lord.

They conceive the fact in this way because their hearts are empty of envy, grudges and pride and their souls are the site of the light of illumination and knowledge.

"…And for those who reject Faith, they ask: ‘What does Allah mean by this parable?'…"
Those who have disbelieved and covered the light of Faith with the curtain of infidelity have ultimately blinded their own insight. They have made their ears, which were able to hearken to the Divine Words, deaf. They, ignorantly or mockingly, inquire that what was Allah's aim by giving this for a similitude that He causes many to err by it and many He leads aright.

In Tafsir-ut- Tibyan, vol. 1, page 19, Shaykh Tusi says: " It is like that a speaker asks what Allah means by this parable that thereby some may be led astray and some be guided. In answer to this question, Allah says that none will be led astray save the transgressors; but the way is open to the believers to reach the climax of understanding and conception with knowledge and true Faith."