A 500 word descriiption of the changes you made, and why. And this “why” should be as in-depth as possible. For example,
if you changed a phrase from “the storm dove down to the top of the sea,” to “the storm wilted down to land softly on the top of the sea,” your explanation of that change should reveal your artistic intention: “I made that change because I wanted the storm to seem like, in actuality, a gentle, well-meaning thing, rather than a harsh thing as is often the association. This also emphasizes and mirrors the narrator’s expressing themself as a misunderstood presence at their ex-partners party…” and so on). Further, imagine how much explanation would then go into a whole paragraph that was exchanged for a new one.
This is perhaps the most straightforward way to think about approaching this assignment: a major revision is so substantial that the actual shape of whatever writing you choose to revise should change visibly. This might mean:
a whole two or three paragraphs get omitted and exchanged for two completely new ones, or
two or three paragraphs are slimmed down, and then two more are added to elaborate or further a plot.
Sometimes it might mean simply that you delete a stanza that isn’t working, or you add a scene in a paragraph or two that ties it all together.
It might mean an over-explaining paragraph or two at the end of a story gets deleted.
A title changes to suggest more and tell less.
It could mean a poem gets broken up into quatrains (four line stanzas), and a stanza is added, and a rhyme scheme is replaced by free verse. Perhaps word choice is refined, etcetera.
Keep in mind: a major revision—while it should be mainly focusing on content and organization—should also include sentence level edits and refining style on a more micro-level, if needed.