Lunar theory is a scientific way of trying to understand how the moon moves around the Earth. Think of the moon as a big ball that is always moving around us. Scientists have come up with ways to predict where the moon will be in the sky at any given time.
They do this by using a lot of math and measurements. They have to figure out how much gravity the Earth has and how much gravity the moon has. They also need to know how fast the moon is moving and where it is in its orbit around the Earth.
All of these things are very complicated, but scientists have been studying the moon for a very long time, so they have gotten pretty good at predicting where it will be. This is important because the moon affects things like the tides in the ocean, and it can also affect how we see things in the sky.
So, next time you look up at the moon, remember that scientists have used lunar theory to figure out how it got there and how it moves around us.