Alright kiddo, so you know how there are rules that everyone has to follow in order to keep things fair and safe, right? Well, sometimes people don't follow those rules and do things that are very bad, like hurting others or stealing.
In a military unit, which is a group of soldiers working together, there are also rules that they have to follow. If a soldier breaks those rules, they can get in trouble and be punished.
A penal military unit is a special group within the military made up of soldiers who have broken those rules and are being punished for doing something wrong. They are sent there as a way to discipline them and give them a chance to make up for what they did.
The soldiers in a penal military unit usually have to do harder and more dangerous work than other soldiers, like clearing land mines or working in war zones. They also may have fewer privileges and be treated differently than other soldiers until they complete their punishment.
The idea behind having a penal military unit is to show that there are consequences for breaking the rules, but also to give the soldiers a chance to redeem themselves and become better soldiers in the future.