Hmm, do you suppose a quick experiment might be faster than waiting for someone to respond? :-)
Trust me, you're going to want to do a lot of experimenting with this program before you have a good feel for how it works; if you don't do that, I wouldn't put much faith in the results.
Here's the definition of priority in the built-in help (Special Columns):
Priority (in task view)
You can manually set a priority on a task, to control the order in which tasks are leveled. Priority can be any integer, so you are welcome to make up your own convention (1 to 3, 0 to 999, or whatever you like). During leveling, if two tasks are assigned to one resource at the same time, the task with higher priority gets to use the resource first. Note that the highest priority task is the one with the largest numerical integer assigned, and negative integers are of lower priority than positive.
I see that the online manual doesn't contain that final sentence, but I think that was an unfortunate editing decision!