What is meant by the likeness of the Qur'an that Allah challenged the disbelievers to produce a surah from? Is it in eloquence or what?

The resemblance intended in the verse is a resemblance in the eloquence of the Qur’an, its sciences, its composition, its descriptions, and its meanings, as well as in its unseen knowledge and truthfulness. It means that the Surah should be similar to the Qur’an in its miraculous rhetoric and style.