Haploinsufficiency is when you have something that is very important, but you only have half of it.
Think of it like having a toy that needs two batteries to work, but you only have one battery. Even though the toy is really important to you, it won't work properly and might not be as good as it could be because it only has half the energy it needs.
Now imagine this is happening inside your body, specifically with your genes. Your genes are like little instruction manuals that tell your body how to work. Sometimes, you will have genes that are really important, but you only have one copy of them instead of two.
This can cause problems because your body might not be able to do everything it needs to do. Just like the toy with only one battery, your body might not function as well because it is missing something really important that it needs to work properly.
Haploinsufficiency can happen in different parts of your body, and it can cause different things to go wrong depending on which gene is missing. Sometimes it can cause diseases, but other times it might not be so noticeable. Scientists are still learning about haploinsufficiency, but they know it can be an important factor in certain diseases and conditions.