The opening lines of "Ode on a Grecian Urn" establish the poem as a work of ekphrasis—that is, writing about an art object. They also quickly set up the poem as a direct address between the speaker and the personified urn. Throughout the poem, the urn fascinates and confuses the speaker in equal measure.
The first four lines show the speaker at a point of relative calm, before the contemplation of the object has provoked any major intellectual dilemma. They represent the starting point of the engagement between speaker and object, and begin with three metaphorical descriptions of the urn.
The basic implication of line 1's metaphor suggests that the urn has an intimate relationship with quietness—that is, quietness is its usual companion. The urn, most likely spending most of its time sitting dormant in a museum, usually exists in a space of non-interaction. It takes on meaning when people look at it, causing them to contemplate its scenes—for this speaker, these scenes go on to bring about thoughts on a wide range of subjects: art, humanity, history, and so on.
That said, museums are often cathedral-like spaces and accordingly these observer-object interactions are often quiet too. The "still" in line 1 functions with two meanings—the urn is "still" because it is inanimate, but it is also "still"—after all this time—wedded to quietness. The use of "unravish'd" suggests that something about the urn remains unconsummated, though as yet the poem hasn't given enough to make clear what that might be—perhaps to be "ravish'd" in this situation would mean to be destroyed, and the speaker is therefore remarking on the sheer amount of time that the urn has survived (somewhere in the region of 2000 years).
The second line develops the idea of quietness by suggesting the urn is somehow parented by "silence" and "slow time." Perhaps it is suggested here that the original parents of the urn were the artist who made it and the contemporary experience the artist was trying to render. Now, the urn is in the care of "silence" and "slow time." The latter phrase also has suggestions of being made of the earth—if the urn is made from clay, for example, then its physical form relies on organic processes that take a long time to develop ("slow" geological time). The sibilance of lines 1-3 is also an attempt to "ssh" the inner ear of the reader, emphasizing the slowness and silence that are being discussed.
Line 3 demonstrates that the speaker begins the poem by feeling that the urn can teach its observers something about the world in which it was made (Ancient Greece). The poem is in large part about whether this statement is true—what can people learn about the past from its objects? The speaker suggests that the urn is better equipped to tell its story than poetry.