Nafs-e-zakiyyah means having a "pure soul". It means having a soul that is clean and good. Just like how we need to keep our body clean by taking baths and brushing our teeth, we also need to take care of our soul by doing good things like helping others, speaking kindly, and being honest.
When we do bad things like hurting others or being mean, it makes our soul dirty. But when we do good things, it makes our soul clean and pure. Having a pure soul makes us feel happy and good inside, and it also helps us to be a better person.
Sometimes it might be easy to do bad things, but if we try to do good things and be kind to others, we will have a pure soul, and that is a very good thing. So always try to do good things and be kind, so that you can have a pure soul!