The French expression "un ange passe" is used to describe a sudden, unexpected break in conversation due to some kind of awkwardness or embarrassment. The question is, is the angel the cause or the effect of the silence? On the one hand, maybe the fact that an angel is passing is what causes the conversation to dry up. On the other, it could mean that the angel is aware of the awkwardness and is passing in order to try to smooth it over.