According to the Lilienfeld text, the facial feedback hypothesis is a theory that we our likely to feel the same emotions that our face is showing. Robert Zajonc hypothesized that when we are making faces that show emotions such as smiling or frowning, those positions change the temperature in our face. These temperature changes "feedback" information to the brain which changes our mood to the corresponding facial feature.
I have my own small connection with this hypothesis. As a cheerleader everyone has always asked me, "how do you stay so happy all the time?" I have always thought that was a very odd question. It has never seemed that hard to me to smile? But how is it that when our team is losing miserably, its 90 degrees and were doing back flips we still leave TCF in a good mood? The facial feedback hypothesis can explain why we stay so peppy during the games, we are smiling through the whole thing. It also explains why that good mood often does not last and about 10 minutes into half time everyone is upset.
The Lilienfled text also proposes an alternative hypothesis that we are classically conditioned to feel that emotion when we are doing the corresponding facial feature. We have learned that happiness is shown with a smile.