A speaker looks back on one ordinary school day that turned into a lasting regret. A friend was sitting alone, so the speaker went over and asked what was wrong. The friend was feeling sick and bad, and when the speaker offered help, he said no. The speaker took that answer at face value and left.
What stands out is how small the moment was. The speaker admits that a regret like this might seem minor next to something "more terrible or horrible," but argues that those can be fixed, and this cannot. The plain, almost conversational language makes the feeling seem honest and unpolished, and the repeated "one" (one regret, one day, one moment) shows how the speaker keeps returning to it. The mood is quiet, reflective, and a little sad. The poem suggests that a friend may need you even when he says he doesn't, and that the speaker has understood this too late.