"Godhead" is a term used across many religions, including Hinduism, Christianity, and the pagan religions of Greece and Rome.
It is easiest to explain it in Hinduism first. Originally, Hinduism was polytheistic. But, as religious and spiritual insight developed, it was seen that, more and more, the many "gods" were really all aspects (or emanations) of One God. This was called the Godhead. The development was from many gods, to many gods with one supreme (a "head god") to the One God being seen as the only God, and the other beings seen as being aspects of the One Godhead.
Similar developments happened in Greece and Rome, with the Sky God, who is also the father of many of the gods (Zeus, Jupiter) being first seen as a Head God (reigning on Mount Olympus) and then, in our understanding, evolving into a Godhead.
Judaism took a different course. In Judaism, the god of the Jewish tribes, YHVH, was declared the only God. Since there were not multiple gods, the term Godhead was not really needed.
Christianity evolved from Judaism by adding the Trinity of God the Father, God the Son, and the Holy Spirit as elements of the One God. This could become confusing, making it seem like there were three Gods. So the term Godhead is also used in Christianity, where it denotes God the Father as the Creator/Father aspect of a single God that includes Son and Holy Spirit.
Even the purely monotheist religions do not see God as having only one aspect. In Judaism, God is the Creator (masculine) and also the Shekhina (Holy Spirit), feminine. In Islam, the one God (Allah) has a hundred names.
The word "Godhead" reminds us of the unity of all that is Divine, even as the Divine reaches us in many forms.