إِنَّ فِي اخْتِلاَفِ اللَّيْلِ وَالنَّهَارِ ...

Verily, in the alternation of the night and the day,

The day and the night alternate, when one arrives, the other goes, and so on, with no errors.

This is similar to the meaning indicated in the following Ayat:

يُغْشِى الَّيْلَ النَّهَارَ يَطْلُبُهُ حَثِيثًا

He brings the night as a cover over the day, seeking it rapidly....

لاَ الشَّمْسُ يَنبَغِى لَهَآ أَن تدْرِكَ القَمَرَ

It is not for the sun to overtake the moon. (36:40)

and,

فَالِقُ الإِصْبَاحِ وَجَعَلَ الَّيْلَ سَكَناً

(He is the) Cleaver of the daybreak. He has appointed the night for resting. (6:96)

Allah continued:

... وَمَا خَلَقَ اللّهُ فِي السَّمَاوَاتِ وَالأَرْضِ ...

and in all that Allah has created in the heavens and the earth,

meaning the signs that indicate His greatness.

This is similar to Allah's statements:

وَكَأَيِّن مِّن ءَايَةٍ فِى السَّمَـوَتِ وَالاٌّرْضِ

And how many a sign in the heavens and the earth... (12:105)

قُلِ انظُرُواْ مَاذَا فِى السَّمَـوَتِ وَالاٌّرْضِ وَمَا تُغْنِى الآيَـتُ وَالنُّذُرُ عَن قَوْمٍ لاَّ يُؤْمِنُونَ

"Say: "Behold all that is in the heavens and the earth,'' but neither Ayat nor warners benefit those who believe not. (10:101)

أَفَلَمْ يَرَوْاْ إِلَى مَا بَيْنَ أَيْدِيهِمْ وَمَا خَلْفَهُمْ مِّنَ السَّمَآءِ وَالاٌّرْضِ

See they not what is before them and what is behind them, of the heaven and the earth. (34:9)

إِنَّ فِى خَلْقِ السَّمَـوَتِ وَالاٌّرْضِ وَاخْتِلَـفِ الَّيْلِ وَالنَّهَارِ لاّيَـتٍ لاٌّوْلِى الاٌّلْبَـبِ

Verily, in the creation of the heavens and the earth, and in the alternation of night and day, there are indeed signs for men of understanding. (3:190)

means intelligent men.

Allah said here,

... لآيَاتٍ لِّقَوْمٍ يَتَّقُونَ ﴿٦﴾

Ayat for those who have Taqwa.

meaning fear Allah's punishment, wrath and torment.