The speaker describes a betrayal, but it isn't the dramatic kind. Trust gets broken "not with a hammer," which would be loud, obvious, and over in a moment. Instead it's undone by "a million itty bitty splinters" pushed slowly into the skin. The damage is made of small, repeated hurts, each one easy to dismiss on its own but painful all together.
The detail that stings most is the "twinkling laugh." The person doing the hurting seems playful, even charming, so the harm is wrapped in lightness. That makes it harder to complain about, and probably harder to forgive. The speaker's tone is quiet and bitter, with a hint of disbelief that something so small and cheerful could do so much harm.
The mood is tense and raw. The childlike phrase "itty bitty" sounds almost sarcastic, as if the speaker is mimicking how the other person shrugged it all off. The title, "Fractures," points to the same idea: something broken in many tiny cracks rather than one clean snap.