To be unbiased, you have to be 100% fair — you can't have a favorite, or opinions that would color your judgment. For example, to make things as unbiased as possible, judges of an art contest didn't see the artists' names or the names of their schools and hometowns.
You are unbiased if you can assess situations with a completely open mind. The root of unbiased is bias, which probably comes from the Greek word epikarsios, meaning “athwart,” “crosswise,” or “oblique.” When you have a bias, you look at the situation “from the side,” such as the side of someone who personally hates seafood telling you that Lobster Larry's is a terrible restaurant. To be unbiased you don't have biases affecting you; you are impartial and would probably make a good judge.