The speaker stands outside a shop window, desperate for a toy they can't afford. At first the longing feels almost like torture, and it quickly turns possessive. When someone else appears with the same gleam in their eyes, the speaker claims the toy as "mine," pounds on the glass, and feels mocked when the other person simply takes it away.
The anger doesn't last. The speaker sighs, admits the glass is too thick and they have no means of getting the toy themselves, and asks what good being mad would do. That calm feels like resignation more than peace, though, because in the last stanza the speaker turns and spots another shop with another toy, and the whole cycle starts again.
The mood moves from frantic and childish to deflated, then ends on a tired, slightly ironic note. Judging by the imagery, the toy could stand for any desire we chase without really thinking, and the poem suggests wanting never quite ends.