Halal is a dietary practice observed by some Muslims.
There are many sects in Islam. Dietary law interpretation may vary from sect to sect. The predominant sect in Iran is Shia Muslims. Shia halal rules generally tend to be more strict than Sunni halal rules.
In general, all plant-based foods are considered halal, as are most milk and eggs. Forbidden are foods derived from pork or any pork derivatives (including enzymes used in dairy products or proteins in gelatin) and alcoholic beverages. Certain seafood is also forbidden, including fish without scales and many shellfish.
Food preparation including processing of allowable meat and other foods must comply with Islamic dietary rules.
You may find halal certification seals on some commercially available food products.