The Quran

وَجَعَلنَا اللَّيلَ وَالنَّهارَ آيَتَينِ ۖ فَمَحَونا آيَةَ اللَّيلِ وَجَعَلنا آيَةَ النَّهارِ مُبصِرَةً لِتَبتَغوا فَضلًا مِن رَبِّكُم وَلِتَعلَموا عَدَدَ السِّنينَ وَالحِسابَ ۚ وَكُلَّ شَيءٍ فَصَّلناهُ تَفصيلًا

Muhammad Asad

And We have established the night and the day as two symbols;¹ and thereupon We have effaced the symbol of night and set up [in its place] the light-giving symbol of day,² so that you might seek to obtain your Sustainer's bounty and be aware of the passing years³ and of the reckoning [that is bound to come]. For clearly, most clearly, have We spelt out everything!⁴

Regarding the primary meaning of the term ayah, see note 2 above. In the present context, the expression ayatayn ("two symbols") refers - as the subsequent clause shows - to the symbols of spiritual darkness and light.
I.e., the message of the Qur'an, which is meant to lead man out of spiritual ignorance and error into the light of faith and reason.
Lit., "the count ('adad) of years". Since, as the Qamus points out, this phrase denotes also "the years of [a person's] life, which he counts", it obviously implies here a call to spiritual self-criticism in view of the ephemeral nature of one's worldly life.
I.e., everything that man may be in need of in the domain of ethics and religion.