We thought he'd been killed, but our prayers were answered when he arrived home safely.

prayers 是名词 prayer 的复数形式，prayer 的意思是"祈祷、祷告"，指向神灵或超自然力量表达愿望或请求的行为或话语。 were answered 是被动语态，表示"被回应"，这里指祈祷的内容得到了实现。