A dialectic is a method of resolving disputes or conflicts between two things. Hegel argued that a dialectical process occurs in society and drives history. He held that two ideas, a thesis and an antithesis, would exist at the same time and would be in conflict. Eventually, the conflcit would be resolved and a synthesis would arise. This process, Hegel said, drove societal change.
Basically, Hegel believed that society was always progressing. He believed that this was happening through the dialectical process. One system of belief or one set of ethical values (thesis) would give rise to an opposing system of beliefs or values (antithesis). The synthesis would be a new set of values and societal change would have occurred. In terms of what you say in your explanation, Hegel believed that society would ultimately evolve to be more and more rational.