What the Prologue Might Mean

In ancient times, the prologue was treated allegorically. Each detail was assigned someprecise allegorical   significance. But, while Parmenides might have had some allegorical intent, I don't think that was his primary thrust. Mostly, I think the prologue served to provide divine sanction for his philosophical vision and to place his philosophical insight outside the "common abidence of mankind." (Heidegger, or at least his translator's phrase.) (it might be worth taking a look at Lucretius' prologue to Venus as a direct imitation of Parmenedes)

There are elements of note:

The verb in the first line is present tense. "The mares carry me. . ." The journey is not something that happened in the past, it is something that continues. (This could also play into the statement later that being is timeless, an eternal present.)

Moving through the gates of night and day places him in the fiery realm, which the physics part of the poem says surrounds all.  In general though, his physics is the least interesting part of the poem. 

There may be some significance in the idea that the daughters of Helios steer the chariot. It may also be important the the goddess justice guards the gate. What that importance might be, though, is unclear to me.

I think one of the goddess' final statements in the prologue is significant--at least for how I understand the philosophy here. She says it is necessary for P to learn all things through asking. She says she will tell him the truth (alethea) but also the common opinion of men and, this is the significant part, why it is necessary that men believe as they do. The mistaken beliefs of men are necessary--not simply ignorance.  

I will talk about this more later, but I think it helps explain why, after describing being as one, unchanging and timeless, he goes on to provide a sort of theogany that describes the origins and nature of things. It is because these beliefs are necessary for men. It is how they must believe to operate in the world.