إِنَّ ٱللَّهَ عَٰلِمُ غَيْبِ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضِ إِنَّهُۥ عَلِيمٌۢ بِذَاتِ ٱلصُّدُورِ · هُوَ ٱلَّذِى جَعَلَكُمْ خَلَٰٓئِفَ فِى ٱلْأَرْضِ
"God knows the unseen of the heavens and the earth; He is the one who knows what the breasts hold within. It is He who made you those who come one after another on the earth."
بِذَاتِ ٱلصُّدُور — "that which the breasts possess", that is, what is kept inside. The same field was treated in Ghāfir (al-Muʾmin) at 40/19 (wa mā tukhfi'ṣ-ṣudūr), and it was recorded there that together with the treacherous glance of the eyes the two ends are covered. I rely on that treatment.
The use of the word here is worth recording: in al-Ḥadīd at 57/7 mustakhlafīna fīh (that over which you have been given charge) was treated, and the trust-character of property was dwelt on there. Here, however, the word describes people's taking one another's place.
And the continuation of the verse ties this to a consequence: fa-man kafara fa-ʿalayhi kufruh — "whoever denies, his denial is against himself." That is, succession is given not as a privilege but as a task whose outcome belongs to the person.
This overlaps with the principle of the burden in the eighteenth verse of the sūra: no one carries another's burden — and here the same thing is said of one's position on the earth.