The idea of roundness and shape is quite important in this poem. The wilderness, sprawling and unkempt, has no defined shape. That's a sharp contrast to the jar, the shape of which is discussed a lot in the poem. So, the poem seems to highlight the differences between the unchecked wilderness and the man-made world, partly by contrasting the shapeless with the shaped.
Line 1: At once when we think of a jar, we see the shape: an immediate sense of roundness comes into our minds.
Line 2: Sure enough, here's confirmation that the jar is round. Not only is the jar round, but it's on a hill, another image of roundness. When we think of round, we think of a smooth, man-made edge.
Line 3: Here, we see that the wilderness is slovenly—wild, overgrown, conforming to no shape.
Line 7: The idea that the jar is round is repeated again here. Now, the poem almost starts to sound round, because of this repetition.
Line 8: Not only is the jar round, but it's tall, and has a presence, and perhaps a function, letting the air come in and out of it like ships in a harbor. Thus, the word "port" has a possible metaphorical meaning. It could be comparing the jar to a port in water. In that case, though, it's a port in air.