The answer is "A group that has concerns about a narrowly defined subject".
A single-issue interest group is a formal organization that has concerns about a narrowly defined subject and aimed to influence policy in order to create, maintain or change that particular subject. Some examples of what a single-interest group are interested in are animal rights, abortion, foreign policy, police brutality, gay rights, defending legal use of guns, etc.
In overall, they aim to achieve its purpose by lobbying government (including members of Congress, the executive branch, etc.), engaging in election activities, educating people about their interests, activities, relevance, current issues, etc., mobilizing people who act on behalf of the organization in order to perform activities, such as writing letters, making phone calls and demonstrating.