The Big Splat Theory is a theory about how our moon came to be.
(Image source: https://en.wikipedia.org/wiki/Giant-impact_hypothesis)
Very simply explained the theorie is that it originally was 2 proto planets, one a little bigger then the other.
In the end they crashed, and from that both Earth as we know it to day and our moon become.
First it created a lot of big rocks that was orbiting around the Earth, and in the end this clumped together and created our moon.
For many millions of years, the moon was much much closer to the Earth then it is now.
And because of that it created Mega storms and weather conditions that made it impossible for life to develop.
But slowly it got further and further away from our planet, until it ended up where it is to day.
(Image source: https://en.wikipedia.org/wiki/Giant-impact_hypothesis)