Ambience is another word for atmosphere in the sense of the mood a place or setting has. If an expensive restaurant has soft lighting and peaceful music, it has a pleasant, soothing ambience.
Ambience is one of those words that we've out and out stolen from French. In French, the word meaning the same thing is, you guessed it, ambience. This is a word you're going to want to know if you have any interest in restaurant reviews — they always want to talk about the ambience. As a funny quirk, the word has a positive feel to it; it can be used to mean nice atmosphere without even using the word, nice, as in, "Hey, this place really has ambience."