Lowering the gaze (ghadd al-baṣar) is an act of spiritual discipline that protects the heart from corruption and preserves modesty. Islam commands both men and women to lower their gaze from what is forbidden, especially from looking lustfully at the opposite gender.
Allah says:
قُل لِّلْمُؤْمِنِينَ يَغُضُّوا۟ مِنْ أَبْصَـٰرِهِمْ وَيَحْفَظُوا۟ فُرُوجَهُمْ ۚ ذَٰلِكَ أَزْكَىٰ لَهُمْ ۗ إِنَّ ٱللَّهَ خَبِيرٌۢ بِمَا يَصْنَعُونَ
"Tell the believing men to lower their gaze and guard their private parts. That is purer for them. Indeed, Allah is well-aware of what they do."
— Surah An-Nūr (24:30)
And to believing women:
وَقُل لِّلْمُؤْمِنَـٰتِ يَغْضُضْنَ مِنْ أَبْصَـٰرِهِنَّ
"And tell the believing women to lower their gaze..."
— Surah An-Nūr (24:31)
The Prophet ﷺ said:
"النَّظْرَةُ سَهْمٌ مَسْمُومٌ مِنْ سِهَامِ إِبْلِيسَ"
"The (lustful) glance is a poisoned arrow from the arrows of Iblis."
— Al-Hakim (Authentic by Al-Albani)
Abdullah ibn Mas'ūd (RA) was known to close his eyes when walking through crowded markets, fearing unintentional glances.
Lowering the gaze protects the heart from desires, strengthens faith, and brings tranquility.