What wikipedia says about lucid dreams:

"A lucid dream is a type of dream where the dreamer becomes aware that they are dreaming. During a lucid dream, the dreamer may gain some amount of control over the dream characters, narrative, or environment; however, this is not actually necessary for a dream to be described as lucid. Lucid dreaming has been studied and reported for many years. Prominent figures from ancient to modern times have been fascinated by lucid dreams and have sought ways to better understand their causes and purpose. Many different theories have emerged as a result of scientific research on the subject and have even been shown in pop culture. Further developments in psychological research have pointed to ways in which this form of dreaming may be utilized as a form of sleep therapy."