The Quran

وَلِلَّهِ غَيبُ السَّماواتِ وَالأَرضِ ۚ وَما أَمرُ السّاعَةِ إِلّا كَلَمحِ البَصَرِ أَو هُوَ أَقرَبُ ۚ إِنَّ اللَّهَ عَلىٰ كُلِّ شَيءٍ قَديرٌ

Muhammad Asad

And¹, God's [alone] is the knowledge of the hidden reality of the heavens and the earth.² And so, the advent of the Last Hour will but manifest itself [in a single moment,] like the twinkling of an eye, or closer still:³ for, behold, God has the power to will anything.

This passage connects with the second sentence of verse 74 - "Verily, God knows [all], whereas you have no [real] knowledge."
As may be inferred from the sequence, the term ghayb - rendered here as the "hidden reality" - alludes in this context to the coming of the Last Hour, the time whereof is known to God alone (Zamakhshari). Parallel with this, it may also relate to God's Own existence, which cannot be directly established by the testimony of our senses (Baydawi) but, as the Qur'an consistently points out, may be inferred from the observable effects of His creativeness.
Lit., "the case (i.e., the manifestation] of the [Last] Hour will be like...", etc. - implying that it will be characterized by utter suddenness and unpredictability, both of them an outcome of the absence of any time-interval between God's decreeing it and its materialization: and this explains the phrase "or closer still" at the end of the above sentence.