It's a common and versatile phrase used in many situations, including:
* Responding to a request or offer: "Can I borrow your pen?" "Mafi mushkil."
* Offering reassurance: "I'm worried about the test." "Mafi mushkil, you'll do fine."
* Downplaying a problem: "I forgot my keys." "Mafi mushkil, we can use the spare."
It's a friendly and informal way to express that something is not a cause for concern.