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 20–21
The present world is full of opportunities, which make it possible for man to do whatever he likes. This leads him into a grave misunderstanding. He considers his temporary freedom to be a permanent condition. The fact is that whatever opportunities man has been given here are by way of trial and not on the basis of his deserving them. As soon as the period of testing is over, all the present opportunities will be snatched away from him. At that time, man will come to know that, except for his helplessness, he possesses nothing which will be a support to him. Man wants to live an unrestricted life. On account of this, he associates beings other than God with the godhead, so that in their names he may be able to justify his misguided actions. But on the Day of Judgement, when reality reveals itself in an unveiled form, he will realize that there was nobody but God who had any powers.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.