In the first sentence of verse 22, it was said: أَفَمَن شَرَحَ اللَّـهُ صَدْرَهُ لِلْإِسْلَامِ فَهُوَ عَلَىٰ نُورٍ مِّن رَّبِّهِ (So I ask about a person whose heart Allah has opened up for Islam, and consequently he proceeds in a light from his Lord.). The word: شَرَحَ (sharh) literally means to open, enlarge or extend. The expression: شرح الصدر (sharh-us-sadr) means the capacity or capability of the heart (to receive and accommodate input). The sense is that one's heart is capable of learning lessons and receiving benefits by deliberating into Divine signs of creation in the heavens and the earth, particularly so by deliberating in his own creation. Similar is the case with other signs of Allah revealed in the form of scriptures and injunctions. One's heart also has to have the ability to deliberate in them and be benefited by them. In contrast, there is a heart that is straightened or hardened. The statement in a verse of the Qur'an: يَجْعَلْ صَدْرَهُ ضَيِّقًا حَرَجًا He makes his heart narrow, much too narrow - Al-An am, 6:125) and the one in the next verse at this place: لِّلْقَاسِيَةِ قُلُوبُهُم "woe to those whose hearts are too hard to remember Allah - 39:22" has appeared in contrast to this very 'sharh-us-sadr' (a heart opened to acceptance of truth and at ease with it).
According to a narration from Sayyidna ` Abdullah Ibn Masud ؓ ، when the Holy Prophet ﷺ recited this verse: أَفَمَن شَرَحَ اللَّـهُ صَدْرَهُ (Is it not that, a person for whom Allah opens up his heart...), we asked him about the meaning of 'sharh-us-sadr'. He said, "when the light of 'iman (faith) enters the human heart, it enhances its capacity (which makes the comprehension and implementation of Divine commandments easy on the person concerned)." We submitted, 'ya rasulallah, what is the sign of it (that is, of 'sharh-us-sadr' )?' Then, he said:
الانابۃ الی دار الخلود والتجافی عن دارالغرور و اتأھب لموت قبل نزولہ ۔ رواہ الحاکم رفی المستدرک ولبیھقی فی شعب الایمان ۔ (روح المعانی)
"Longing passionately for the eternal home, and seeking refuge from the deceptive abode, and preparing for death before its arrival." - Reported by al-Hakim in al-Mustadrak and al-Baihaqi in Shu` ab-u1-'iman (Ruh-ul-Ma’ ani).
The verse under study has been initiated as headed by an interrogative particle: اَفمَن (afaman). The sense it carries can be explained by saying: 'Can a person whose heart has been opened up for Islam - and he is on the light coming from his Lord, that is, does everything under it - and another person with a hardened heart be equal?' The contrasting part relating to the hardened heart has been mentioned in the next verse with a warning of woeful punishment attached to it.
In this verse (22), it was said: فَوَيْلٌ لِّلْقَاسِيَةِ قُلُوبُهُم (woe to those whose hearts are too hard to remember Allah). The word: الْقَاسِيَةِ (al-qasiyah) is a derivation from: قَسَاوَت (qasawah) which means to be hard-hearted, having no mercy for anyone, and also the one who remains totally unaffected by the need to remember Allah and follow His injunctions.