Your daughter says of your son: "He hit me."
She has made an allegation.
Assume that the evidence shows that there is a red mark on her arm, tears and she and her brother were the only two people in the room at the time.
This evidence is enough to establish a prima facie case.
However, this evidence has not yet been tested.
Now, for obvious reasons, we are going to assume that your family operates on an inquisitorial rather than an adversarial model of justice of so it is you who will be doing the testing.
You might ask for a statement of the fact from your son. This will almost certainly contradict the evidence of your daughter because ... siblings. Assume that your son says they were arguing over a controller and that your daughter threw it at his head, he ducked and it bounced off the wall and struck her in the arm.
So now you might cross-examine both the plaintiff and defendant to try to find any inconsistencies or other reasons to doubt their testimony. Or you might examine the controller for evidence of it being smashed into a thousand very expensive pieces.
At the end of this process, you may prefer your daughter's version over your son's: if so, your daughter has met her burden of proof and is entitled to whatever remedy your Solomonic wisdom decrees. However, if you prefer your son's or find them equally plausible then she hasn't and justice requires a remedy in the other direction. This is because it was your daughter who had the burden of proof throughout.
However, assume instead you son said: "I hit her but she hit me first". Well now he has admitted to the alleged facts and your daughter's burden is met but he is raising an affirmative defense ("I did it, but ...") rather than a negating defense ("I never did!"). The onus has now shifted to him to demonstrate the required elements of self-defense. Of course, unlike in the public legal system, self-defense is generally not a total defense withing a familial legal system and the best he can hope to do is mitigate the punishment and, vitally important in the interests of justice for children, ensure his sister gets punished too; because vengeance is more important than mercy.