Mainly, sociology undertaken from an idealist perspective is concerned with ideas. Idealists hold that what is important about society is ideas. Societies, they say, really only exist if people think they exist. Therefore, it is extremely important to study what people think about their societies. This way of thinking is associated most notably with Hegel.
This sort of thinking is very much in conflict with Marxist, materialist ways of thinking. Materialists believe that everything is tangible and that everything important is based on material culture, not on ideas. This is another way to think of idealism--to see that it is in direct contrast with Marxist perspectives on sociology.