Derivatization is like putting a costume on a molecule. You know how when you put on a costume, you can look and act like a completely different character? That's kind of what happens when we derivatize a molecule.
When scientists study molecules, they need to be able to see and measure them. But sometimes molecules won't let us do that very easily because they're too small or too similar to other molecules. That's where derivatization comes in. Scientists put on costumes (derivatizing agents) onto the molecule to make them easier to study.
For example, if scientists wanted to study a molecule with a lot of carbons, they might derivatize it with a molecule that has a special tag on it that makes it easier to find. Or if scientists wanted to study a molecule that's usually a gas, they might derivatize it to make it into a liquid so it's easier to handle.
Once the molecule is wearing its costume (derivatization), scientists can study it and learn more about its properties and behavior.