The Quran

وَالَّذينَ يُظاهِرونَ مِن نِسائِهِم ثُمَّ يَعودونَ لِما قالوا فَتَحريرُ رَقَبَةٍ مِن قَبلِ أَن يَتَماسّا ۚ ذٰلِكُم توعَظونَ بِهِ ۚ وَاللَّهُ بِما تَعمَلونَ خَبيرٌ

Muhammad Asad

hence, as for those who would separate themselves from their wives by saying, "Thou art as unlawful to me as my mother", and thereafter would go back on what they have said, [their atonement] shall be the freeing of a human being from bondage¹ before the couple may touch one another again: this you are [hereby] exhorted to do - for God is fully aware of all that you do.²

I.e., the freeing or purchasing the freedom of a slave or captive. In modern times, when slavery is more or less non-existent, the concept of tahnr raqabah may, I believe, be legitimately extended to the redeeming of a human being from the bondage of debt or of great poverty.
Cf. 2:225 - "God will not take you to task for oaths which you may have uttered without thought, but will take you to task [only] for what your hearts have conceived [in earnest]".