It's only fitting that an epitaph would concern life and death, as, by definition, an epitaph is a short text written in honor of someone who has died. This poem ties in some wisdom about life and death in the process. You could say to someone who hasn't read this poem that it is, indeed, a matter of life and death that they read it.
Title: By saying that this poem is an epitaph, the poem lets us know immediately that the woman is dead, without having to spend time on how or why she died. The word "epitaph" also helps us to imagine a gravestone, because, often, epitaphs are found engraved on tombstones.
Lines 2 -3: The word "dust" symbolizes the husband's dead body in these lines. It could also literally mean that he's been dead for so long that he has decayed into dust, but realistically, this would take so long that it's probably not entirely possible.
Line 4: Just as it's probably not possible for the woman's husband to be dust, it's also impossible for that dust to tremble. This is an example of personification, where the dust is given human capabilities. This gives the reader a physical way to imagine the husband as he is joined by his wife in death. Instead of just telling us what is going on, the speaker shows us through using an impossible scenario.
Lines 5-6: This is the part of the poem where the message about life and death is neatly packaged for us—but there's still plenty of room for interpretation. The colon makes these lines seem like a definition. Really, though, they're an explanation of the first part of the poem and a statement about life. The optimistic way to read them is to think that the reason why death is so sad is because life is truly wonderful, and the pessimistic way is to think that all joy in life results in sorrow at death. Which way do you choose? Or would you rather walk the middle line?