It is speech which is concentrated and directed for a clear purpose. To reveal character through what each character says and how they say it. To advance the plot by giving important information to the audience. Good dialogue can tell the audience about things they don't see as well as clarify the things they do see.
Exposition is the process of giving information to the audience.
Since the purpose of expository dialogue is to present facts, it must be clear. Also, this information is best learned through the action of the story. Keep it motivated so it doesn't simply seem like exposition to the audience. It is difficult for an audience to comprehend too many different ideas which are clustered together in the same speech. For very important details, bring up the idea three times or more so your audience does not miss the point.