The Oxford English Dictionary indicates the word can be used as an adjective, adverb, or noun. Shakespeare uses it as a noun, with the meaning that another responder has already provided. The OED also says that the word, in Shakespeare’s time, “was a more dignified word than now.” Beyond that, in the context of the play, hurley-burley, with the meaning of battle, suggests that someone will win and someone will lose (because such are the dynamics of warfare). The words immediately introduce verbal irony because the day will signal a great loss and a great victory (thus, as elsewhere in the play, the witches equivocate), and also situational irony, for both the victory and defeat are of an unforeseen and surprising nature. It can also be noted that the word “hurley-burley” provides a good example of onomatopoeia, a word which suggests its meaning.
A hurley-burley is an uprising or conflagration usually relating to war.
However, I like the response of critic L.C. Knight who argues:
It is worth remarking that "Hurley-burley" implies more than the "tumult of sedition or insurrection." Both it and "when the Battaile's lost, and wonne" suggest the kind of metaphysical pitch-and-toss that is about to be played with good and evil. At the same time we hear the undertone of uncertainty: the scene opens with a question, and the second line suggests a region where the elements are disintegrated as they never are in nature; thunder and lightning are disjoined, and offered as alternatives. We should notice also that the scene expresses the same movement as the play as a whole: the general crystallizes into the immediate particular ("Where the place?"—"Upon the Heath."—"There to meet with Macbeth.") and then dissolves again into the general presentment of hideous gloom. All is done with the greatest speed, economy and precision. (32-33)
When shall we three meet again
In thunder, lightning, or in rain?
When the hurlyburly's done,
When the battle's lost and won.
That will be ere the set of sun.
Where the place?
Upon the heath.
There to meet with Macbeth. (I.i)