A ticket’s priority is simply a ranking (from 1 to 3) of how urgent or important it is.

But what does it do, and how should I use it?

Out of the box, ticket priority doesn’t actually do anything. However, Zammad administrators can set up all sorts of automated hooks that fire off based on this value, like:

Priority can also be used as a ticket filter when creating custom overviews.

In other words, consult your administrator for details on how he’d like you to use it.