In short, it's a dream you can know you are in a dream. In a lucid dream, besides knowing it's a dream, you also can apply your knowledge, experiences and logics to a lucid dream. And then manage your dream, turn nightmares into sweet dreams or end the dream as you wish. Think about it, when we have sweet dreams with the abilities about knowledge and logics, we must can remember more things in our dreams and get more happiness after waking up. Think more, if we can end nightmares with ease, fearness will keep us away.