What does all hail Macbeath hail to the thane of Glamis mean?

All Hail is used primarily as a greeting, specifically a greeting to someone of a higher social order. A thane is kind of like a Lord and Glamis is a place. Saying all Hail Macbeth, Thane of Glamis is just acknowledging the title that he holds. When the witches say that he is already the Thane of Glamis.