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).

**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”?](https://literature.stackexchange.com/q/13886/2191)). 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](https://www.bartleby.com/70/4113.html)) 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.