An interest group is essentially a group that advocates for some sort of common interest and promotes it in a way that seeks to influence government and policy in favor of the group's interests.
Being aware of the agendas of interest groups as well as their ratings on politicians as well as candidates is important to understanding the powers that can influence elected officials' stances and votes.
See how elected officials vote (particularly members of Congress) at the following links. This can help you tie together interest groups and votes to see the potential influence of interest groups on American foreign policy.