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),
- You shall be (or become) what you have been promised. This refers to the prediction by the Third Witch in Act I, scene 3: "All hail, Macbeth! that shalt be king hereafter."
The verb "be" in "and shalt be" here takes a subject complement, and this subject complement is on the next line: "What thou art promised" (i.e. what you have been promised).
Update:
Note that "shall" (as in "shalt be") can mean "will" ("you will be king") but also "will inevitably or assuredly, be bound to, must" (see A Shakespeare Glossary by C. T. Onions, revised by R. D. Eagleson, 1986). When the witches say "thou shalt be king hereafter" and Lady Macbeth mimics these words, there is more going on than a simple prediction: the words also have overtones of unavoidable destiny.
From Lady Macbeth's point of view, Macbeth's kingship is something that requires effort, and even an effort to overcome whatever humaneness exists in Macbeth's character (see the related question What kind of language features appear in Lady Macbeth's line “too full o' the milk of human kindness”?). Whether the witches use "shall" in the sense of "will" or "will inevitably or assuredly" is ambiguous and is interpreted differently by different characters. Macbeth's initial reaction (Act I, scene 3) in incredulity: "and to be king / Stands not within the prospect of belief", whereas Lady Macbeth begins to think of means to turn that prediction into reality.