Alcohol by volume (ABV) is like how much juice is in your cup of water. You know when you pour some juice into a cup of water, the juice makes the drink stronger, tastier, and different than just plain water. Similarly, ABV tells us how much alcohol is in a drink.
When grown-ups drink something with alcohol, like beer, wine, or mixed drinks, they usually want to know how strong it is. ABV tells us the percentage of alcohol in that drink.
So, if we had a drink that had 10% ABV, it means that for every 100 parts of the drink, 10 parts are alcohol.
Just like how too much juice in your water can be too strong, too much alcohol in your drink can be too strong too. That's why grown-ups often check the ABV of the drink before drinking, so they know how much alcohol they are putting into their body.