When there is unanimity, everyone agrees. When it comes to opinion, unanimity is the opposite of disagreement.
Unanimity is a word for a group of people all being of one mind. If everyone in the country voted for the same person for president, that would be complete unanimity. Just like uniforms make people look alike, this similar word indicates people are thinking alike. Unanimity doesn't happen often, because people tend to disagree about everything. When there is disagreement, there's no unanimity.