Every object has a natural frequency that they vibrate at (called resonant frequency). In the case of a glass container, you can hear its resonant frequency by tapping it with a spoon. Remember, as a child, you must have used a swing set. To make the swing go higher and higher you give it a nudge as soon as it reach its highest point. Basically, you are magnifying the swing by giving it a nudge at the right moment. That's what opera singers (or in this case the woman in the magic show) they raise their pitch to be the same frequency as the glass' resonant frequency. The energy in the voice acts as the nudge (in the swing example above) and makes the magnitude of the vibration larger and larger. The glass breaks when the vibration becomes strong enough (note that glass is not a very elastic substance). A very famous example of an external stimulus at the natural frequency of an object causing it collapse is the Anger's bridge (https://en.wikipedia.org/wiki/Angers_Bridge). The cadence at which the soldiers were marching matched the resonant frequency of the bridge. It caused the vibration to get stronger and the bridge to collapse. Read more about resonant frequencies of bridges here - https://www.livescience.com/34608-break-stride-frequency-of-vibration.html

A woman can break a glass cup with her voice cause glass cups can resist that much preasure. Moderator's comment: This answer is not entirely correct and can be improved further.