Daena is a concept in the Zoroastrian religion which refers to a sort of guardian spirit or angel that is assigned to every person at birth. It's kind of like having a special imaginary friend who is always with you, but this friend is supposed to be helping you make choices that will lead to a good life.
The idea is that your daena is always watching over you and trying to guide you towards making good choices. It's like having a little voice inside your head that tells you what's right and wrong, and helps you make decisions that will keep you on the right track.
In Zoroastrianism, it's believed that your daena will continue to be with you even after you die, and will help guide your soul to the afterlife. So it's kind of like having a lifelong guardian angel who will always be by your side.
Overall, the idea of daena is meant to help people live good and moral lives, by reminding them that they are never truly alone and that they always have someone looking out for them.