祭祖 is a verb-object compound (离合词) referring to the traditional Chinese practice of offering sacrifices, food, or incense to deceased ancestors, especially during Qingming Festival or the Ghost Festival. The verb 祭 and noun 祖 can be separated: you can insert aspect particles like 了/过 or measure words like 次 between them (e.g., 祭了一次祖 'made an offering once'). It's a deeply cultural act rooted in filial piety, and you don't place another object after it; the 'ancestor' is already part of the compound.