1. Just what did the prophets see and know? As one of them put it, G-d “spoke to the prophets, (and also) gave them many visions and related parables through them” (Hosea 12:11), in the course of dreams and while they were awake. That’s to say that the prophets were granted visions that served as images, metaphors, similes, symbols, or representations of G-d’s ways in the world. The exception to that was the prophecy of Moses, the greatest prophet of all, to whom G-d spoke “mouth to mouth; in a vision (of the literal truth) and not in riddles” (Numbers 12:8).
But the visions that the prophets had weren’t the products of rigorous intellectual insights or of profound reckoning. Their insights came to them from up above rather than from deep within. And they understood that what was granted them was a sure and doubtless revelation rather than something that could be argued with or countered. For not only did they receive figures of speech that would express what they needed to say to the people, they were also granted sure insight into its meaning and significance, and into the fact that it was something that G-d Himself wanted to pass along.
2. Understand, though, that the prophets were privy to different sorts of images overall: those that depicted various aspects of G-d’s “personality” in the world, if you will; and those that depicted His actions.
There would be times, for example, when G-d wanted to express His beneficence and mercy to a prophet, so He’d seem to appear as a kindly elder (see Daniel 6:9); and other times He wanted to express His might against those who seem to thwart His wishes, so He’d appear as a warrior (see Mechilta, Yitro 20:2; also see Chagiga 14a for both examples).
Examples of images that speak to His actions in the world — which were often arcane and perplexing — include Jeremiah’s almond-staff (Jeremiah 1:11) and the boiling pot (Ibid. v. 13), Jacob’s ladder (Genesis 28:12), Ezekiel’s scroll (Ezekiel 3:1), Zachariah’s measuring basket (Zachariah 5:6) as well as his golden Menorah-lamp (Ibid. 4:2), etc. The import and underlying message behind these sorts of images weren’t obvious to the prophet at first any more than they were to the others who experienced it, but G-d soon clarified them .
 For kabbalistic references to this chapter see R’ Shriki’s note 179
 See Derech Hashem 3:4: 1-2 as well as chapter 7 of Rambam’s Hilchot Yesodei HaTorah for a discussion of all this.
Rabbi Yaakov Feldman has translated and commented upon “The Gates of Repentance”, “The Path of the Just”, and “The Duties of the Heart” (Jason Aronson Publishers). His works are available in bookstores and in various locations on the Web.