All Muslims, Shia and Sunni, fast during the month of
Ramadan (Ramazan), i.e. they eat something, called Sahari, before Azan
of morning prayer (about 1:30 hour to sunshine) and then don't eat
anything during daylight hours until Azan Maghrib (about 15 minutes
after sunset). They also must not drink, have intimate relations, swim,
etc. After Azan Maghrib, they break their fast (Iftar).
Now, the question is Why do Muslims Fast during Ramadan?
Simple answer is that it is Wajib(obligatory) in Islam for healthy adults according to Quran(2:183):
O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous -
However, it has several advantages which some of them are described in Hadith of Miraj in where the prophet Muhammad(pbuh) ascended to the heaven and talked with Allah:
The Prophet Muhammad (pbuh) asked Allah: O God! What is the start of worship? He said: start of worship is fasting and silence. My Lord! What is the effect of fasting? Said: fasting leads to wisdom and wisdom leads to knowledge and knowledge leads to certainty; and when a person comes to the position of certainty, it’s no matter, his life is easy or difficult
Part of Hadith Meraj, BiharAlanvar, V 74, P 27
Arabic and Farsi translations