The phrase c'est la vie is French, meaning "that's life." In English, it's usually used when something mildly unfortunate or disappointing occurs, showing a kind of philosophical resignation.
Saying "c'est la vie" is a bit like a verbal sigh. You might say it when it rains on the Saturday when you planned a picnic, or when you spill coffee on your favorite shirt, or when you just miss your bus by a few seconds. These events are unfortunate, but certainly not tragic, and there's really nothing you can do about them but shrug and move on.