Okay little buddy, have you ever played with Legos and built something cool? Open G tuning is kind of like that, it's a way to change how you build your guitar chords so they sound different.
When we play guitar normally, we use what's called standard tuning where the strings are tuned to these notes: E A D G B E. But open G tuning is when we tune our guitar's strings to a different combination of notes, it goes like this: D G D G B D.
So why would we want to change the way we tune our guitar? Well, when you play in open G tuning, it makes certain chords easier to play and gives your guitar a different sound. It's like building Legos in a new way, you can create things you couldn't before.
For example, in standard tuning, playing a G chord can be a little tricky, but in open G tuning, all you have to do is strum all the strings together and you've got a G chord. Plus, the open strings make the guitar sound kind of twangy and country-like.
So, in summary little buddy, open G tuning is like building a new Lego creation with your guitar. It changes the way your guitar chords sound and makes some chords easier to play.