al-Tayyib and al-Khabīth are interpreted in three senses:
1. al-Tayyib is used to mean lawful (ḥalāl), and al-Khabīth to mean unlawful (ḥarām), as in the following verses:
Do not exchange the tayyib with the khabīth, that is, do not exchange your lawful wealth with {the unlawful}! (al-Nisāʾ 4:2)
Say: "Even if the abundance of the khabīth impresses you, the khabīth and the tayyib {that is, the unlawful of people's wealth and the lawful} are not equal." (al-Māʾida 5:100)
Intend/turn towards a tayyib {that is, lawful} purification! (al-Māʾida 5:6)
Marry from the tayyib {that is, lawful} women for you! (al-Nisāʾ 4:3)
2. By al-Tayyib and al-Khabīth, the believer (muʾmin) and the disbeliever (kāfir) are intended, as in the following verses:
Allah will not leave the believers in the state you are in until He separates the khabīth from the tayyib {that is, the people of kufr from the people of iman}. (Āl ʿImrān 3:179)
So that Allah may separate the khabīth from the tayyib {that is, the people of kufr from the people of iman}... (al-Anfāl 8:37)
The vegetation of a tayyib land {that is, fertile land} comes forth by the permission of its Lord. But that which is khabīth {that is, barren and arid land} does not come forth, and even if it does, it is of no use. (al-Aʿrāf 7:58)
This is the parable of the believer and the disbeliever. The tayyib [fertile] land/soil resembles the believer; when he hears it, he benefits from it with iman. The khabīth land/soil resembles the disbeliever; even if he hears it, he does not benefit from it with iman.
3. The word al-Tayyib means beautiful, as in the following verse:
To Him ascends the tayyib word (that is, the beautiful word—which is the testimony that there is no deity except Allah), and righteous action raises it. (Fāṭir 35:10)
Likewise, a righteous person acts in this way.
Do you not see how Allah presents an example: a tayyib word (that is, a beautiful word: the testimony that there is no deity except Allah)? (A tayyib word) is like a good tree, whose root is firmly fixed and its branches are in the sky. (Ibrāhīm 14:24)
Similarly, if the believer testifies that there is no deity except Allah and performs righteous deeds, his word and deed ascend to Allah.