Advertising often serves as an indicator for what society considers to be socially acceptable and desirable at the time,” US marketing expert wrote on the Ad Council’s website.

【翻译】美国营销专家在广告委的网站上写道：“广告往往是现时社会认为社会可接受并可取的一种标志。”

what 作为“融合性关系代词”，引导名词性从句作动词或介词的宾语，是英语最常见的现象之一。

本句的 what society considers to be socially acceptable and desirable at the time，作介词 for 的宾语。同时what 本身又是该从句consider 的宾语，to be socially acceptable and desirable at the time 作宾补。