When your mind is on something, you think about it or give attention to it.

I couldn't concentrate on my work - my mind was on other things. 我无法集中精力工作——我的心思在别的事情上。

My mind wasn't on what he was saying, so I missed half of it. 我的心思没在他所说的话上，所以恐怕漏听了一半的内容。