A moderate voter is someone who is politically speaking, in the center. In other words, they are not quite left or right, but somewhere in the middle.
Moderate comes from the Latin for "medium sized," and as a noun and an adjective it means "middle, medium." If you get moderate rainfall, it's not too much and not too little. Goldilocks was known for her moderate tastes. As a verb, it means "to temper," or "to reduce from the extreme." If you moderate your speed while driving, you slow down a bit. If you moderate a discussion, you help two or more people have a reasonable conversation.