The speaker describes opening up completely to someone they love, and being badly hurt for it. Their bones turn to sand and then to glass, a image of becoming transparent and breakable. Inside this glass body sits a glass heart that once "beat so vibrantly" and shone through the chest. The speaker invites the other person in, showing them what they had always protected with a layer of stone so it would never fall apart.
That trust doesn't last. In the final stanza the tone drops into quiet grief: the speaker is silent, crying into a pillow, with a heart now "a shattered mess" scattered inside a hollow chest. The poem never says exactly what went wrong, which keeps the focus on the feeling itself. The mood is fragile, sad and intimate, and the repeated word "glass" makes the speaker's vulnerability feel almost physical. It reads like a private confession after heartbreak.