Such flags are mostly used by Shia Political & Military Groups in Iran, Iraq & Lebanon.

La ilaha ilallah

La ilaha ilAllah (Arabic: لا إله إلا الله‎, lā ʾilāha ʾillallāh) is part of the Muslim proclamation of faith or šahādah. The Arabic phrase translates as: There is no deity except God.

Lā لا means “no”
ʾIlāh إله means “deity”
ʾIllā إلا means “except”
Allāh الله means “God”.

It is the first part of the Islamic creed known as the kalimah or šahādah.

Ahadith mentioning virtues of this phrase

Ǧābir ibn ʿAbd-Allāh reported that Muhammad said: "The best remembrance of Allah is to repeat lā ʾilāha ʾillallāh and the best prayer (duʿāʾ) is al-ḥamdu li-llāh (all praise belongs to Allah)." (Narrated by an-Nasāʾī, Ibn Māǧah, and Ḥākim an-Nisabūrī who declared its chain sound)