At first, the whole idea of a poem being silent and "wordless" may not make sense to us. But once we think a little more about the notion of a poem "being" without "meaning," the whole silence motif makes a bit more sense. So even though it's a kind of paradox, a poem being wordless and all, it's also kind of the big point of "Ars Poetica."
Lines 1-2: So a poem should be "palpable and mute" like a "globed fruit." We should feel it but we shouldn't hear it screaming truths into our ears. In other words, it's more important to "sense" a poem than to "know" it's so-called meaning.
Lines 5-6: A poem should be "silent" like those casement ledges all worn by too much elbow action. So it should be natural and effortless in a way, not trying to resist time or be consumed by it. It should be more like the moss that's growing along the window ledge, never shouting at us.
Lines 7-8: A poem should be "wordless" too like a "flight of birds." We see the beauty of the birds' formation, but we usually don't hear them. So we sense the beauty without feeling bombarded by it with a lot of noise, and that's how a poem should read.