There are several meanings of 人气. The most common meaning of this word is 'popularity' or 'popular' nowadays, which comes from entertainment industry in Japan. You can say 人气歌手, in which 人气 is adjective. You can say 人气很旺, in which 人气 is noun. But in literary contexts, the meaning of 人气 is more likely 人的气息, the breath of human beings. You can imagine a place where more and more people gather around and breathe the air, the place is becoming more and more popular (many people pay a visit).