Okay kiddo, so imagine you have a big glass of lemonade that can be really sour or not so sour. Let's say you want to make it a certain amount of sourness - not too sour and not too mild. This is kind of like what the Henderson-Hasselbalch equation does, but with chemicals instead of lemonade.
You see, there are some molecules called acids that have a lot of hydrogen atoms in them. When these acids are in water, they can give away their hydrogen atoms and become "deprotonated". This makes the water more acidic. But there's another side to this - when the water is too basic, some of these acids will grab onto those hydrogen atoms and become "protonated". This makes the water less acidic.
The Henderson-Hasselbalch equation helps us figure out how much of each we have in a solution. It uses some fancy math things, but the basic idea is that we can look at the pH (which tells us how acidic or basic something is) and the pKa (which tells us how likely an acid is to lose its hydrogen atoms) to figure out how much of the acid is deprotonated or protonated.
Just like how you might add a little bit of sugar to your lemonade until it tastes just right, scientists can use the Henderson-Hasselbalch equation to calculate how much acid or base they need to add to a solution in order to get it to the right pH. And just like how you might have to taste your lemonade a few times to make sure it's perfect, scientists might have to do some experimenting to get their solution just right.
So, in short, the Henderson-Hasselbalch equation helps us figure out how much acid and base we need to add to a solution to get it just the right level of acidity.