The technical details were over my head. [=too complicated for me to understand]那些技术细节我理解不了。

That joke went right over my head. [=I did not get that joke]那个笑话我没听懂。

We realized after we started the business that we were (in) over our heads. [=we were trying to do something that was too difficult]我们在开始做这件事之后才意识到我们能力有限，无法完成。