The memory being described is a man or boy remembering his father looking in on him while he thought he was asleep.
The son in the poem is remembering something that occurred several times over a period of years. When he was small, his father used to look in on him while he was sleeping. His father apparently used to strike a match so that he could see his son without waking him up.
Half in dreams, he saw his sire
With his great hands full of fire.
Since the match lasts only for a moment, the boy thinks of it as a “secret heart.” It is a metaphor for his father's love. The match flame looks like a heart in his hands, and the father is not supposed to know he is watching. Since this is the son’s dominant memory of his father, we can infer that they did not have a particularly close relationship, and the secret spying helped the son know his father loved him.
the son's memory is described. he's telling the story of his memory of his father