I believe that the aim of true theatre is to reconcile us with a certain idea of action, of immediate effectiveness.

红色部分为介词短语，作 a certain idea of action 的非限制性定语，前有逗号说明是非限制性的。它是修饰整个a certain idea of action 词组的。意思是说，这种行为理念“具有立竿见影的效果”。

of + 抽象名词作后置定语（也可作表语），说明被修饰名词具有的某种品质。

It's a matter of great importance. I would advise you to think carefully. 事关重大，请你三思。
This is a picture of great value. 这是一幅很有价值的画。

我重新翻译一下：

I believe that the aim of true theatre is to reconcile us with a certain idea of action, of immediate effectiveness.

【翻译】我认为，真正的戏剧之目的是让我们符合一定的行为观念，且有立竿见影之效果。