The circled U or OU symbol on food labels is a kosher trademark indicating that the product is in accordance with Jewish dietary laws. The mark helps consumers keep a kosher kitchen when lacking firsthand knowledge of food processing procedures.
The Orthodox Union of Jewish Congregations in New York City owns the OU trademark. They state, "OU Kosher is the world's largest and most widely recognized kosher certification agency, certifying more than 500,000 products produced in over 6,000 plants located in 80 countries around the world."
In order to qualify for the OU mark, companies must adhere to the strictest level of kosher compliance. All ingredients and processes used in the preparation of the food must be kosher for the end product to meet kosher standards. For example, an otherwise kosher food is disqualified if coated in animal-based glycerin or prepared on equipment that also processes non-kosher preparations.
One of Many
The OU symbol is only one of many kosher trademarks that you may find on food labels. Each certifying agency has its own qualifying standards for using their symbol. See the resources for a detailed list of kosher trademarks to look for.