So amber is hardened tree sap, and it's really, really good at preserving stuff. Throughout this poem, Pound looks for different images to convey art's power to preserve beauty over hundreds or even thousands of years. So it's natural that amber would be one of those images, as Pound writes in line 230, "As roses might, in magic amber laid." So he's talking about something beautiful (a rose) being put in amber so its beauty can last forever.
Amber is pretty incredible stuff, after all. Just look at what it can preserve. Those bugs aren't going anywhere, and they'll stay in perfect condition for thousands, maybe millions of years. For Pound, beauty is something that's definitely worth preserving, and the image of amber does a good job of symbolizing the power of art to preserve beauty.
- Line 230: Pound compares art preserving beauty to hardened amber preserving a rose. It's a pretty straight-up comparison, but a good one.