How does voting on ideas affect my reputation?

As with other things you do in the community -- creating content, responding to discussions, and so on -- your actions on ideas contribute points to your overall status in the community. (If you aren't familiar with your status, you can view it on your profile page.)

You might be getting status points for the following (you'd typically get more points for creating them than commenting or voting on them):

Creating ideas.
Commenting on ideas.
Voting on ideas.