Decisions which affect the whole world are made behind closed doors and are top secret.

behind 的意思是"在...后面"，这里引申为"不公开地"。 closed 是形容词，意思是"关闭的、封闭的"，表示"不对外开放的"。 doors 是 door 的复数形式，意思是"门"。 behind closed doors 是一个固定短语，字面意思是"在关着的门后面"，引申为"私下地、秘密地"，指不公开进行的活动或决策。