Rashi: They should not have erred to imagine that dreams are Nevu'ah. When Nevu'ah comes to a Navi, it comes with strength, like burning fire - "v'Hayah v'Libi k'Esh Bo'eres" (20:9), "v'Yad Hashem Alai Chazakah" (Yechezkel 3:14).

Radak: Dreams are vain. You see that they say Shalom, and there is no Shalom! However, My word is like fire that goes out and burns what it will find, and like an ax shatters a rock. Nothing stands in front of My word to nullify it. What I said, I will do; no matter will fall (unfulfilled).

Malbim: This is the second of the three ways to recognize whether something is Nevu'ah or a dream. Fire burns and spreads light in a dark place. So a word of Nevu'ah casts flashes of the fire of Hashem's flame in the power of the Nefesh, and illuminates the light of the Nefesh.

Ma'alos ha'Torah (DH Isa b'Sefer): Just like fire burns thorns, so Divrei Torah burn all bad Midos in a person, which are compared to thorns - "k'Kotz Munad

Rashi: It is an ax.

Radak: Refer to 23:29:1:2

Malbim: This is the third way to recognize whether something is Nevu'ah or a dream. Just like an ax shatters a rock to pieces ¹ , so [Nevu'ah] shatters the body and nullifies its powers. The knees [of a Navi receiving Nevu'ah] knock.