Sensitization is when you become more sensitive to something after being exposed to it many times.
Let's say you love eating spicy food. At first, you might only be able to handle a little bit of spice, but over time, as you eat more spicy food, your tongue might get used to it and you might be able to handle even more spice. This is sensitization.
However, sensitization can also happen with things that aren't pleasant or fun. For example, if you get a shot from the doctor, the first time might be scary and painful. But if you get shots many times over a long period, your body might start to get used to the pain and the scary feeling. This means that you might need a bigger or stronger dose of medicine to have the same effect as before.
So, sensitization is when your body or brain changes in response to something that you experience repeatedly. Sometimes this is a good thing, and other times it can be a bad thing.