When you shatter something, you smash it or break it into small bits. If your sister hits a baseball directly into the kitchen window, the window will shatter — and your sister will be in big trouble.
When you drop something fragile, it will probably shatter—lightbulbs, teacups, mirrors, and windows all shatter fairly easily. Things can also shatter in a less literal way, when it just feels like they're breaking apart violently. If your best friend ignores you all day, your heart shatters; and if you stumble and fall on your way up to a podium to give a speech, your confidence may shatter.