A person who feels let down by both home life and work has found a strange way to cope. By day, the speaker counts ships, a quiet, almost numb routine, the kind of thing you do when you want your mind to hold still. It sounds like a variation on counting sheep, a way of passing time when nothing feels worth doing.
Then the poem turns darker. At night come fires, and the speaker admits to having taken "great pleasure" in many of them. The title helps here: it plays on "schadenfreude," the guilty delight in other people's misfortune, and gives it a Freudian twist, as if this pleasure comes from somewhere buried. The poem doesn't say whether the fires are real or imagined, and that uncertainty adds to the unease.
The mood is flat, confessional and slightly chilling. Short lines and a calm tone make the admission feel more disturbing. The speaker's disappointment seems to leak out as a quiet appetite for destruction.