Yawma tara almu'mineena walmu'minati yasAAa nooruhum bayna aydeehim wabi-aymanihim bushrakumu alyawma jannatun (surah Hadid: 12)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(57:12) On that Day you will see believing men and women that their light will be running before them and on their right hands. *17 (They will be told): 'A good tiding to you today.' There shall be Gardens beneath which rivers flow; therein they shall abide. That indeed is the great triumph.

Yawma taral mu`mineena walmu`minaati yas`aa nooruhum baina aydeehim wa biaymaanihim bushraakumul yawma jannaatun tajree min tahtihal anhaaru khaalideena feeha; zaalika huwal fawzul `azeem

*17) This and the following verses show that the Light on the Day of Judgement will be specifically meant for the righteous believers only. As for the disbelievers and the hypocrites and the wicked people, they will be wandering about in the darkness as they had been in the world. The light there will be the light of righteous deeds. The sincerity of the faith and the piety of the character and conduct will turn into light that will lend brightness to the personality of the virtuous. The brighter the deed the more luminous will be his person, and when he will walk towards Paradise, his light will be running forward before him. The best explanation of it is Qatadah's mursal tradition in which he says; "The Holy Prophet (upon whom be Allah's peace and blessings) said: "The light of some one will be so .strong and sharp that it will be running on before him equal to the distance between Madinah and `Aden, of another equal to the distance between Madinah and San'a, and of another even less than that; to much so that there will be a believer whose light will Dot extend beyond his steps." (Ibn Jarir). In other words, the intensity of the light of a person will be proportionate to the extent of the good done and spread by him in the world, and the beams of his light will be running on before him in the Hereafter extending as far as his good will have extended in the world.
Here, a question may arise in the mind of the reader "One can understand the meaning of their light running on before the believers but what does their light running on only on their right hand mean? Will there be darkness on their left side? The answer is: When a man is walking with a light on his right hand, his left side also will be bright, though the fact of the matter is that the light will be on his right hand. This has been explained by the Hadith, which Hadrat Abu Dharr and Abu Darda' have reported, saying that the Holy Prophet (upon whom be Allah's peace;) said: "I shall recognize the righteous people of my Ummah by their light which will be running on before them and on their right and on their left." Hakim, Ibn Abi Hatim, Ibn Marduyah).