"Halal" is an Arabic word that means "permissible" and it's typically used to describe what foods are acceptable to eat for Muslims. Non-Muslims most likely see the word halal applied to meat and other foods.
Properly prepared halal meats must come from animals that are healthy when they are slaughtered. The slaughter must take place in a specific manner that includes draining all of the animal's blood. Certain foods and beverages are considered to be categorically not halal, including pork and alcoholic beverages in addition to any meats that are not prepared according to halal rules.