祠堂 is a clan ancestral hall or temple — a physical building where a Chinese family lineage (clan) worships its ancestors, stores ancestral tablets, and holds ceremonies during festivals like Qingming. It's a culturally significant institution in traditional Chinese society, not just a religious site but a social and legal center for the clan. The word is a place word (p.w.), so it can directly follow prepositions like 在 without needing a measure word.