Thank you for the opportunity to help you with your question!
Each p orbital can hold 2 electrons. It is a natural tendency for the electrons to move as far apart from each other because of their charge. So each p orbital is filled with one electron first. We have 1 electron in each p orbital. If we add another electron, it will have to pair up with one electron in one of the p-orbitals.
It is more stable to have a half-filled p sublevel, where there is one electron in each p orbital, than if it was not half-filled.
Note that an atom having a filled or half-filled sublevel is slightly more stable than an atom with some other arrangement. (This fact should be in your chemistry book.)
Please let me know if you need any clarification. I'm always happy to answer your questions.