What is meant by "divorce is twice"?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. It clarifies the Sunnah method of divorce, which is to issue only one divorce within one period of purity. [Ibn ʿAbbās, Mujāhid]
2. It explains the limit within which the husband retains the right to take his wife back after divorce (i.e. after the first or second divorce). [ʿUrwah, Qatādah, Ibn Qutaybah, al-Zajjāj, and others]