It is what powers the giant advertising and marketing industries in their relentless attempts to map and exploit our cognitive and emotional vulnerabilities.

It 是人称代词，意为“它；这；那”；what 是融合性关系代词，没有疑问概念。what 本身指“某物；某件事”，既是先行词，又同时做了关系代词。所以，what = the thing that/which。

又如：

What surprised us was his behavior.

= The thing that  surprised us was his behavior.

让我们惊讶的事情是他的举止。

What is in the box is a jar, and what is in the jar is some water.

= The thing that is in the box is a jar, and the thing that is in the jar is some water.

盒子里的东西是一个罐子，罐子里的东西是一些水。

【翻译】就是此种力量推动了广告和营销行业的巨头们不懈地探索和利用我们的认知和情感弱点。