Should resentment or antipathy arise between husband and wife, he is not to divorce her but to bide in patience throughout the course of one whole year, that perchance the fragrance of affection may be renewed between them. If, upon the completion of this period, their love hath not returned, it is permissible for divorce to take place. God’s wisdom, verily, hath encompassed all things. The Lord hath prohibited, in a Tablet inscribed by the Pen of His command, the practice to which ye formerly had recourse when thrice ye had divorced a woman. This He hath done as a favour on His part, that ye may be accounted among the thankful. He who hath divorced his wife may choose, upon the passing of each month, to remarry her when there is mutual affection and consent, so long as she hath not taken another husband. Should she have wed again, then, by this other union, the separation is confirmed and the matter is concluded unless, clearly, her circumstances change. Thus hath the decree been inscribed with majesty in this glorious Tablet by Him Who is the Dawning-place of Beauty.

The Lord hath prohibited . . . the practice to which ye formerly had recourse when thrice ye had divorced a woman.
This relates to a law of Islám set out in the Qur’án which decreed that under certain conditions a man could not remarry his divorced wife unless she had married and been divorced by another man. Bahá’u’lláh affirms that this is the practice which has been prohibited in the Kitáb-i-Aqdas (Q&A 31).

He who hath divorced his wife may choose, upon the passing of each month, to remarry her when there is mutual affection and consent, so long as she hath not taken another husband . . . unless, clearly, her circumstances change.
Shoghi Effendi states, in a letter written on his behalf, that the intention of "the passing of each month" is not to impose a limitation, and that it is possible for a divorced couple to remarry at any time after their divorce, so long as neither party is currently married to another person.

Question: Concerning the sacred verse: "The Lord hath prohibited . . . the practice to which ye formerly had recourse when thrice ye had divorced a woman."
Answer: The reference is to the law which previously made it necessary for another man to marry such a woman before she could again be wedded to her former husband; this practice hath been prohibited in the Kitáb-i-Aqdas.