وَمَآ أَدْرَىٰكَ مَا ٱلْعَقَبَةُ
This pattern occurs in about thirteen places in the Qurʾān and always does the same work: a word is uttered, then there is a pause, then it is described. It was treated in Sūrat al-Humaza (wa mā adrāka ma'l-ḥuṭama), and the same structure is present in such sūras as al-Qadr, al-Qāriʿa and al-Ḥāqqa.
The function of the pattern is to create a pause. The narrative is cut while it is flowing; the attention of the addressee is gathered; then the description comes. For an oral text this is a technique for preparing the hearer.
Root: د-ر-ي. To know a thing by investigating, by following its trace. Dirāya — comprehension. The causative of the verb, adrā — he made known.
A consistent distinction in the Qurʾān: the pattern mā adrāka (what made you know — perfect) is used in places where an explanation follows. The pattern mā yudrīka (what makes you know — imperfect) is used in places where no explanation follows, where the knowledge is left closed to man (for instance about the hour of the resurrection: al-Aḥzāb 33/63, "What will make you know? Perhaps the Hour is near").
This distinction has been recorded in the classical commentaries and is consistent within the Qurʾān itself. Here adrāka is used — that is, an explanation is to come. And indeed the next verse begins the description.
One point has to be seen: the question is not about something unknown. The word ʿaqaba is a word everyone who knows Arabic knows; what a steep ascent is, is plain.
The answer lies in what the question is aimed at. What is asked is not the dictionary sense of the word but what that ascent corresponds to. And the addressee's guess on this point is most likely wrong.
For the things that come to mind at the words "steep ascent" are known: long acts of worship, night prayers, fasts, austerity, withdrawal, self-restriction. When hardship is spoken of, this is always what comes to mind in the religious life.