The statement that Reverend Hale makes is quite powerful because he equates the assumption of Rebecca's supposed guilt with regard to witchcraft to the extraordinarily exaggerated idea of "the whole green world burning." His metaphoric representation is used to illustrate just how far-fetched and improbable the notion of her being guilty of such a transgression actually is. The fact is that something green does not burn easily and, in addition, the hyperbolic idea that the whole world can burn emphasizes just how outlandish the suggestion is.
Furthermore, what the reverend is alluding to is that if Rebecca Nurse is suspected of witchcraft, then no one can be free from any accusation and, consequently, everyone, no matter what their standing or background, can then be charged. The word green also suggests innocence and a lack of knowledge and further accentuates that everything has been befouled by witchcraft's evil touch.
Reverend Hale makes the above statement because he emphatically believes in Rebecca's innocence. Her benevolence is known as far away as his hometown, Beverley, and when he sees her for the first time, he recognizes her immediately. This is only possible because she projects an image of benevolence and the reverend must have only been receiving positive testimony about her wholesome goodness. He tells Rebecca on their meeting:
It's strange how I knew you, but I suppose you look as such a good soul should. We have all heard of your great charities in Beverly.
Rebecca and her husband are two of the foremost and most respected and admired citizens in Salem, and they have received much gratitude and appreciation from its inhabitants. Rebecca, as can be seen from her actions in the play, has great wisdom and intelligence and only seeks to help. She has a soft and gentle demeanor and is clearly committed to doing good.
It is tragic that Reverend Hale's protests come to naught, for Rebecca is, just like so many other innocents, eventually hanged because the true malice pervading the village is not discovered but given free rein to exercise its perfidy—ironically from the platform of righteousness.
I think that Hale's meaning is quite poignant. It is one of the first moments in which the audience sees Hale as offering some small level of dissent against unconditional acceptance of the accusations. Hale comes to know Rebecca Nurse as a paragon of virtue and care. His initial introduction to the town is one in which he singles out Rebecca Nurse as the embodiment of compassion. Hale knows this to be an absolute part of her character.
When she is accused, it is one of the first times that Hale begins to question what is transpiring in Salem. Hale understands clearly that Rebecca Nurse's accusation reveals one of two realities. The first is that the devil's presence in Salem is far greater than his capacity to counter it if it can corrupt a woman like Rebecca Nurse. The other is that there is something wrong with the accusations and the people making them if Rebecca Nurse can be found guilty.
It is here where Hale begins to make a distinct change in how he views the girls' accusations. His statement about how if Rebecca Nurse is guilty, the "whole green world" is one that is left "burning" is symbolic of this corruption. For Hale, the accusation of Rebecca Nurse is significant because it begins to trigger a process through which the perception of purity and "green" is contrasted with a more jaded sense of "burning" and corruption.