In just three short lines, the speaker describes a small, almost funny mishap that turns out to feel quite sad. They once felt safe and comfortable inside a box, probably the kind with a combination lock, as the title "safety box" suggests. But they forgot the digits, and now they are shut out of the very place that made them feel protected.
The box can be read literally, but it seems to stand for something larger: a state of comfort, a relationship, a memory, or a sense of security that the speaker can no longer get back to. The poem doesn't say how the code was lost, which makes the loss feel quiet and ordinary, the kind of thing that just happens.
The mood is wistful and a little helpless. The plain, flat tone suggests the speaker isn't shouting about it, just accepting it. The awkward phrasing of the last line, "out i am locked," makes the lockout feel even more final.