It literally translates to "not of what" but it's used in the same way as "you're welcome" in English, as a polite response to someone expressing gratitude.
Here are some examples of how to use "pas de que":
* Person A: Thank you for helping me with that.
* Person B: Pas de que.
* Person A: Merci beaucoup pour le cadeau.
* Person B: Pas de que.
* Person A: Thanks for the ride!
* Person B: Pas de que.
It's a common and widely used expression in French.