The flag of Yemen (Arabic: علم اليمن) was adopted on May 22, 1990, the day that North Yemen and South Yemen were unified.
The flag of Yemen is essentially the Arab Liberation Flag of 1952, introduced after the Egyptian Revolution of 1952 in which Arab nationalism was a dominant theme. The Arab Liberation Flag served as the inspiration for the flags of both North and South Yemen prior to unification, and the current flags of Egypt, Iraq, Sudan, and Syria.
According to the official description, the red stands for the bloodshed of martyrs and unity; the white for a bright future; black for the dark past. Wikipedia