A speaker notices a ghost girl drifting nearby, someone no one else seems able to see. Instead of running, they sit beside her and ask gentle questions: who she is, what she needs, whether they can help. The feeling is closer to loneliness than fear. The speaker is alone too, and the ghost seems to reflect that.
The poem's most interesting moment is the line about crosses, which repel ghosts, yet the speaker holds on tightly to her anyway. So the usual horror rules are turned around: the ghost isn't the threat, and the speaker wants her to stay. One small change near the end shows this. The speaker first asks "Can I stay" and later "Can you stay," as if the need goes both ways. The final line, "I'll stay, I'll stay, I'll stay," repeats like a promise. The mood is quiet, tender and a little sad, a poem about offering company to someone who feels invisible.