The word "concern" has several meanings, depending on its context.
1) Generally speaking, a concern is something that causes worry or anxiety; it can also refer to an issue, problem, or matter that someone is thinking about or considering. For example, one might say, “I have concerns about the new policy.”
2) In business, "concern" refers to a company or organization, especially when used in conjunction with another word (e.g., "food concern").
3) In a more specific sense, "concern" can refer to a particular field of activity or interest; for example, someone might say they have a concern about the environment, meaning that it is an issue that they care deeply about and want to see addressed.