好家伙 is an interjection used to express strong emotion — surprise, admiration, disbelief, or even mild indignation — depending on tone and context. It's roughly equivalent to 'good heavens!', 'wow!', or 'what a...!' in English. You'll hear it in casual speech when reacting to something unexpected or impressive. Unlike many interjections, it can be followed by a full sentence describing what triggered the reaction.