Alright kiddo, so you know how grown-ups love drinking grape juice that's been left to sit for a really long time until it's turned into alcohol? That whole process is called winemaking, and oenology is just a fancy word for the science of winemaking. It's a lot like how some people love making cupcakes and follow very specific recipes to make sure they turn out perfectly, except instead of cupcakes, oenologists only care about making sure the wine they make tastes great.
Now, different types of grapes can make different types of wines, and oenologists can use their knowledge to choose the right grapes for the kind of wine they want to make. They can also use different techniques to ferment the grapes, add certain ingredients to change the flavor, and even age the wine for years to give it a nicer taste. Think of oenology like a chef making a fancy dinner, except instead of cooking, they're focused on making great-tasting wine.