The speaker begins by describing how "you"—that is, anyone—occasionally hear stories about someone running away to start a new life. This is the type of "departure" to which the poem's title refers.
From the poem's start, though, the speaker seems skeptical about the idea of suddenly chucking it all. For one thing, the speaker says that you never hear the story directly. That is, you never actually talk to someone who has actually departed from their old life. It's never even told second- or third-hand but rather "fifth-hand," casting doubt on the idea that anyone ever follows through with this fantasy of escaping and starting again. The further a story is from its actual source, the harder it is to trust it is true (a bit like the game of telephone).
The speaker compares this archetypal story to an "epitaph," a commemorative phrase of the kind normally found on tombstones. These "fifth-hand" stories, in a way, memorialize the person who has disappeared, treating the breakaway from employment, friends, family, and so on as so deep and permanent that it's as if that person has died. The point of the story is to make "you" marvel at how someone can just kill off their own life with such daring.
Notice how casually lines 3-4 describe this disappearance:
He chucked up everything
And just cleared off,
Both "chucked up" and "cleared off" are colloquial British expressions, and both denote a kind of willing indifference. That's part of what's supposed to be so impressive about the story: the way people can dismiss their own lives as if they mean nothing. That heavy /uh/ assonance in "chucked up" makes this departure seem all the more sudden and abrupt.