Lucid dreaming is what happens when you become aware that you're dreaming while you're dreaming. Normally, we wake up and think "wow, that was a dream." But it can be really fun and interesting to wake up while you're still dreaming and realize "wow, this is a dream!"
Once you wake up in your dream, the entire reality is under your conscious control. How fun it that? You can do anything you want. You can have crazy good dream sex! You can fly. You can become bigger or smaller. You can create a whole new reality. You can go to outer space. You can dig deep into The Divine and ask to see The Spirit. You can bring back loved ones who have left the physical realm. You can meet aliens from other galaxies.
Many ancient cultures use dreaming as a spiritual practice. Among them are the Tibetans, Native Americans, ancient Egyptians and Chinese Taoists. Australian Aborigines see the whole physical reality as a dream and life as Dream Time. Over the last hundred years, modern scientists, philosophers, artists and plain folks have combined ancient wisdom and western critical thinking to create a wealth of knowledge on the subject. You can really dive deep into the practice; there is a lot of information out there, much to learn and a lot of fun to be had.
Dreaming is a part of our brain's normal functions. While we sleep, the parts of our brain that govern logical and analytical functions are not operating at waking capacity. Images we gathered from our waking life come to life without too much interference or consideration from our habitual behaviors. In a way, we get a chance to "create" reality, so we can explore our created reality differently. What I believe to be real becomes "real" immediately. I feel dreams are a gift from my eternal self, my hidden observer. There are two aspects of myself, one is of the body and one is of the spirit or to put it in another way, the ego and the hidden observer. While I dream, one part of my brain is engaged in the dream and I learn to use the other part of my brain to observe.