The words "Glamis thou art, and Cawdor; and shalt be / What thou art promised" can be explained as follows:

* You are Thane of Glamis (Macbeth's title at the beginning of the play),
* You are Thane of Cawdor (a title conferred on Macbeth after the original Thane of Cawdor defected to the invading Norwegians; see [Act I, scene 2](https://www.bartleby.com/70/4112.html)),
* You shall be (or become) what you have been promised. This refers to the prediction by the Third Witch in [Act I, scene 3](https://www.bartleby.com/70/4113.html): "All hail, Macbeth! that shalt be king hereafter."

The verb "be" in "and shalt be" here takes a [subject complement](https://www.grammar-monster.com/glossary/subject_complement.htm), and this subject complement is on the next line: "What thou art promised" (i.e. what you have been promised).