Fitrah sister is the human nature. Human nature for example, knows what is good and what is bad. It is disgusted with bad behavior and leans towards good behavior and actions or if you have done something bad you feel bad about it. This human nature goes way deeper than my simplistic explanation, as also part of this human nature is already acknowledging that there is 1 God. However when a person is born, his/her parents teach him/her things for example that go out of their way and follow things that are not in line with human nature. The more a person follows Islam in the actions, the more the fitrah becomes clean from "dirt" so to say. The more practicing you become and thus more the fitrah becomes pure, the less you want to look at haram things or talk bad about people or even listen to people talking bad about other people.
