This poem's aging speaker begins by highlighting their self-sufficiency and independence in a confrontational apostrophe to an unknown "you." The "you" they're addressing in these lines might be a specific younger person, a group of younger people, or younger people in general. From its very first lines, the poem as a whole warns younger people not to stereotype, patronize, or underestimate their elders.
In particular, lines 1-4 warn against the assumption that the speaker (and, by extension, seniors in general) are always lonely, and need constant conversation to comfort them. If they're "sitting quietly," the speaker says, it's because they want to be alone with their thoughts: it's because "I'm listening to myself." They're not secretly hoping someone will keep them company, and they don't need anyone else's "chattering" to cheer them up.
Though they're part of a larger stanza, these first four lines work as a rhymed quatrain, with an ABCB pattern. But readers shouldn't get too comfortable with that pattern: it will change and shift throughout the poem, and so will the meter! Like the speaker's personality, the verse will be a little rough around the edges, and will sometimes defy expectations.
The simile "Like a sack left on a shelf" shows the speaker's willingness to poke fun at themselves. They know they look as plain as a "sack" at this stage of their lives, and might appear lonely or forgotten, as if "left on the shelf." (But that doesn't give others the right to make assumptions about them.) Their blunt self-deprecation helps gain the reader's trust while adding a twinkle of humor to their proud, defiant tone.
At the same time, the humble words "sack" and "chattering" (linked with /a/ assonance) contrast with the more elevated phrase "listening to myself." It's as if the speaker's saying: I may not look like much, but I'm more thoughtful than you realize—and probably more interesting than you!