A voting cue is something that helps someone know how they should vote on a particular issue. It is often used with regard to members of legislatures, but can also refer to the voting behavior of members of the general public.
In legislatures, members often rely on cues to tell them how to vote so they do not have to truly become informed on issues. This saves them the time and effort that would be needed to educate themselves on an issue that may not be of great interest to them or even to their constituents. Such legislators will often rely on cues like how important colleagues are voting, what trusted interest groups' attitudes are, or what their party leadership wants them to do. These are the voting cues that allow them to decide how to vote.
Similarly, individual voters may have cues. They may rely on other individuals whose opinions they trust. They may rely on the candidate's party affiliation. They may rely on the endorsements of interest groups that they trust, such as the NRA or the AARP.
In short, then, a voting cue is some piece of information that helps a person decide how to vote on an issue or in an election without having to become fully informed on the issues or the candidates.