It can be used in a few different ways depending on context:
* To describe the location of an object: "El libro está en mi escritorio" (The book is on my desk).
* To indicate the place where work is done: "Estoy trabajando en mi escritorio" (I'm working at my desk).
* To describe a state of being busy or occupied: "Estoy ocupado, tengo mucho trabajo en mi escritorio" (I'm busy, I have a lot of work on my desk).
So, the exact meaning of "en mi escritorio" can vary depending on how it's used in a sentence.