Idealism is a philosophy developed by Plato. One of the many tenets of idealism is that truth can be found through reasoning, intuition, and divine revelation. There is such as thing as absolute truth and the world is composed of ideas primarily.
For curriculum, idealist concepts come through when people believe that learning is mostly an intellectual process. Teaching connects ideas together when teaching the students. The education is highly structured and one of the best examples of this is the liberal arts education. The humanities are viewed as the most important subjects because these fields deal with ideas. The sciences are lower on the scale because they deal with observation.
Many famous educators supported idealism. Among them includes William Harris, Fredrich Froebel, and William Bennett. A more detailed analysis of idealism and teaching is available here.