A lethal dose is the amount of something that, if you have too much of it in your body, will cause you to die.
Let's use the example of medicine. If you have a headache, your mommy or daddy might give you some medicine to make it go away. The medicine has a certain amount of the medicine inside of it that is safe for your body to handle.
If you take too much medicine, you can get very sick and it can even be dangerous. The amount that is safe for your body is called the dosage.
If you take more than the recommended dosage, it can be lethal, which means it can kill you. So it's very important to always listen to the instructions on the medicine bottle and not take more than what it says, even if your headache won't go away right away.
That way, you can stay safe and healthy!