Sawm — fasting in the month of Ramadan — is the fourth of the Five Pillars.
"Islam is built upon five [pillars]: … fasting in Ramadan."
— Sahih al-Bukhari 81
Why Muslims fast
Fasting is prescribed not as hardship for its own sake, but to cultivate taqwa — God-consciousness and self-restraint:
"O you who have believed, decreed upon you is fasting as it was decreed upon those before you, that you may become righteous."
— Quran 2:183 (Saheeh International)2
The month of Ramadan
Ramadan is honored above other months because it is when the Quran was first revealed:
"The month of Ramadan [is that] in which was revealed the Quran, a guidance for the people."
— Quran 2:185 (Saheeh International)3
What the fast involves
From dawn until sunset, Muslims abstain from food, drink, and marital relations, then break the fast in the evening.678
"And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the night."
— Quran 2:187 (Saheeh International)4
The same passages make clear that those who are ill or traveling are excused and may make up the days later, reflecting Islam's principle of ease.3
The Night of Decree
Within Ramadan falls Laylat al-Qadr, the Night of Decree, described as greater than a thousand months:
"Indeed, We sent it [the Quran] down during the Night of Decree. … The Night of Decree is better than a thousand months."
— Quran 97:1-3 (Saheeh International)5
The month ends with the celebration of Eid al-Fitr.
In short
Fasting in Ramadan is a month of discipline, devotion, and empathy — abstaining from dawn to sunset to draw closer to God, honor the month the Quran was revealed, and remember those who go without.