Okay kiddo, so you know how some people believe in a big powerful god called Yahweh, right? Well, in some ancient cultures, they had special people called priests who would talk to Yahweh and do things to make him happy.
One thing that these priests wore was a special gold plate on their head. This plate was shiny and round and had special things written on it. They would tie it onto their forehead with a pretty ribbon.
The golden head plate was very important because it showed that the priest was special and chosen by Yahweh to talk to him. It also had some magic powers that would help the priest do his job.
So, in short, the priestly golden head plate was a fancy gold plate worn by priests in ancient times to show that they were chosen by God and had special powers to do their important job. And, it's because it was quite important to their religion.