Hello, guys. can someone explain the meaning of the poem "to keep one's treasure protected"

Within the lump of coal the flame lies hidden,
within its darkness hides the diamond’s glory:
all unseen from without, it must be imagined—
the fire that heats the house, the wedding ring
sparkling with future promise. But what would be
the coal’s choice if coal could be said to choose?
All day I have been trying to imagine the ones
who withhold themselves—arms folded across chests,
or hands buried deep in their pockets. The ones
who remain a few steps back from life, who feel
possessed of a treasure which they don’t wish
to offer the world, as if they wore their smiles

on the insides of their faces. Is this an attempt
to save themselves for the truly important moment?
Or could it suggest the world isn’t good enough?
Or are they trying to be complete in themselves—
both lover and loved, consumer and consumed,
as if one could be complete without the world?

What does it mean never to offer, not necessarily
to be selfish but never generous, as if afraid
to spill a valuable part of the self, something
not seen as golden until it is gone, as if Self
were a red bird that one squeezes in the hands
thwarting its wish to fly off into the pine trees?

Those withholders in the doorway, those lumps
of coal who flee the fire: to see a man slip
to the sidewalk without going to help, to know
a song and not sing it, to watch the hungry
get hungrier, the defeated continue their steady
collapse. Our bodies are coinage. Spend it. Fling

the coins upward, hear them jangle on the street.
What happens to the souls of the miserly?
A man creeps down to his basement at midnight,
digs a hole, unearths a box, unfastens a lock.
Inside, a little dust, a spider, two lumps of coal.
A sigh—isn’t it like a scream turned inward?