To burst someone's bubble is to reveal something to them that they were unaware or ignorant of, often in the form of bad news. However, it doesn't necessarily have to be something negative (see example 2).

When someone thinks they are the main character of their life. Usually comes with a side of individuality complex, quirky style and a self centered point of view.

When someone is feeling all happy and opptimistic, you take your asshole self and totally break his happyness, therefore bursting his bubble. Eventually, you will become known for bursting people's bubbles, and they will hate you with a burning passion.

Fart in the bathtub and bite the bubbles. Congratulations, you've just physically bitten a fart.

When someone pokes there finger into you, usually your stomach. Some idiots will say "Burst you bubble"