The speaker is talking about their own head, and they're not kind about it. It's "a most troublesome fellow," always aching, always reaching. What it wants is huge: to take the whole world and "knot it up," as if the world were a loose rope or tangle of thread that could be tied into something manageable. The title, "Grandiloquent," suggests big, showy talk, so there's a hint that the speaker knows this ambition is a bit absurd.
The last two lines shift the tone. The mind that wants to control everything looks at "the disorder of desire" and sees it differently, as "a wing for love." Messy wanting becomes something that lifts rather than something that gets in the way. The mood is wry and self-teasing at first, then turns softer and more hopeful. The poem seems to say that restlessness isn't only a problem. It might be what lets love fly.