Ever look up at the moon? Ever been entranced by what you see? It's this massive rock, floating thousands of miles away form the Earth. It orbits around us, and based on where it is in regards to the sun and us, it's anywhere from full, to seemingly nonexistent. It effects the tides, gives us light, and has often been the source of dreaming. It was one giant leap for mankind when we set foot on the moon 42 years ago.
I remember, back when I worked at a movie theater, I would have to bike to and from work. It was about a 20 minute bike ride down the bike paths. When I closed, or when I would watch a movie afterwards, I would have to bike home in the dark. They didn't put lights on the bike trails like they did on roads, so when it was 11 o' clock, or later, it was a pretty dark trip. I always enjoyed when there was a full, bright moon because it made it easier for me to see, and I didn't have to worry as much about any obstacles that my be in front of me. I wasn't such a big fan of the nights when we had new moons, and it was very dark. It made me weary of going too fast, so it would take me longer to get home.
Think of our preexistent state as day time. God is present, giving us light and warmth, and we have no hindrance to our vision. We can see everything just fine. However, when we come to this world, it becomes as night. We are no longer in the presence of our Heavenly Father, and have much greater difficulty seeing what lies ahead. But fortunately, God won't just leave us to our own devices. Rather, he sends us a wonderful gift: prophets. Prophets are like the moon: they receive light from the sun (God) and give it to the people. Thanks to prophets, even on dark nights, they give us enough light that we are much better able to see the path ahead of us. Throughout history, people had accepted and rejected the prophets, in an ongoing cycle of apostasy and dispensations. We can think of this as similar to the lunar cycle. A prophet is introduced to the people, and they listen to his words and heed his counsel. This is like the waxing of the moon, as it grows towards its full state. Eventually, it is full, and the people prosper.
However, it doesn't take long for people to start disliking being told what to do, or what they're doing wrong. So, as so often seen, the prophets were rejected, persecuted, and sometimes killed. The people lost that light and guidance. This can be compared to the waning period of the moon, as it begins to get smaller, until it reaches the new moon. The new moon is an apostasy, when that light and guidance is gone.
How lucky we are to be in the Dispensation of the Fullness of Times. We have that full, bright moon that gets light from God, and gives it to all of us to help us along in our journey. We should always follow the counsel of the prophets, because he will always guide us in righteousness.