This Ayah offers a way out for husbands. If a husband has accused his wife but cannot come up with proof, he can swear the Li`an (the oath of condemnation) as Allah commanded. This means that he brings her before the Imam and states what he is accusing her of. The ruler then asks him to swear four times by Allah in front of four witnesses
إِنَّهُ لَمِنَ الصَّـدِقِينَ
(that he is one of those who speak the truth) in his accusation of her adultery.
(And the fifth; the invoking of the curse of Allah on him if he be of those who tell a lie.) If he says that, then she is divorced from him by the very act of this Li`an; she is forever forbidden for him and he must give her Mahr to her. The punishment for Zina should be carried out on her, and nothing can prevent the punishment except if she also swears the oath of condemnation (Li`an) and swears by Allah four times that he is one of those who lied, i.e., in what he is accusing her of;
The fourth injunction is of Li'an (لِعَان : curse) among the adjuncts of fornication
The meaning of لِعَان and is to curse and praying for Allah's wrath on each other. Certain specific types of oaths between husband and wife with special connotation are called Li` an (لِعَان) in Islamic law. When a husband accuses his wife of adultery or refuses to own his child as being legitimate, and his wife refutes his allegation to be false, and claims for the punishment of false accusation (قَذَف) of eighty stripes to be awarded to him, then the husband will be asked to produce four witnesses. If the husband produces four witnesses, then the wife will be awarded the punishment of adultery, and if he could not produce four witnesses, then they will be subjected to li'an (لِعَان), that is first the husband will be asked to testify four times with the wordings given in the Qur'an to the effect that he is honest, and the fifth time will say that if he was lying, then Allah's curse be on him.
If adultery is declared a serious crime, it follows, logically, that falsely accusing a person of adultery must be considered a serious crime too. Therefore, it is laid down that one who accuses another of adultery without being able to prove it, (as prescribed by the rules of the Islamic law), should be whipped eighty times. Moreover, his evidence should be treated thereafter as entirely unacceptable, and, according to the Hanafi School of jurisprudence, the evidence of such a transgressor may never again be legally accepted, even if he repents. Levelling false allegations against anyone is, in fact, an attempt at moral assassination. Severe punishments are prescribed in Islam for such an offence. Even if a wrongdoer escapes punishment in this world, he cannot in any case escape punishment in the life Hereafter, unless he repents and seeks God’s pardon.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.