A concept album is an album that has a unifying idea, or some greater theme that the rest of the album fits into. So for example, Pink Floyd's Animals, which has tracks named after certain animals, and the animals are used as metaphors for things people do. "Dogs" being somewhat about the rat race, competition in business and life, etc.
Another example would be Deltron 3030, which is set in the year 3030 and is about the adventures of deltron and his sidekick dan the automator (ie producer and rapper), in the year 3030.