In Assassin's Creed Origins, Bayek of Siwa is a Medjay.
A Medjay was an elite protector and law-enforcement figure in ancient Egypt, a role that had developed from the Medjay people of Nubia. In the game, Bayek serves as the Medjay of Siwa, carrying responsibilities that include protecting his community and responding to threats against local people.
Origins is set during the first century BCE, near the end of the Ptolemaic period. Bayek and Aya become involved in a conspiracy that eventually leads to the formation of the Hidden Ones, the organization that later becomes the Assassin Brotherhood. The game therefore presents an origin story for the series' fictional order.
Medjay is not a synonym for pharaoh, soldier in every context, or ordinary town guard. Bayek's title connects his personal identity to a much older Egyptian institution, while the game's later Hidden Ones represent a new organization created in response to the events of the plot.