In verse 19, it was said: يَعْلَمُ خَائِنَةَ الْأَعْيُنِ (He knows the treachery of the eyes), in other words, eyes that betray the trust. It means the action of a person who would, secretly and surreptitiously, cast a glance over something haram and impermissible for him or her, for example, casts a glance at a non-mahram person with sexual desire, and takes it away in the event someone was around, or casts a glance in a manner that is not noticed by others. All these things are open before Allah Ta’ ala.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 17–19
There are countless signs in the universe which teach us lessons in symbolic language. One of these is the system of rains. This natural phenomenon is symbolic of God’s inspiration. Just as rains are useful for fertile land and useless for barren land, similarly the inspiration from God brings forth fruit in some but not in others. This ‘rain’ enters the souls of those who have kept their hearts open and makes their existence lush green. On the contrary, those whose hearts are full of the greatness of beings other than God are like barren lands. God is fully aware of His subjects. Whoever He finds capable, He chooses for the purpose of conveying His message. The main aim of this message is to warn the people of the fast approaching day when they will be presented before the Lord of the Universe, from whom nothing will be hidden and whose judgement will not be influenced by anybody.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.