Theres been speculation about priorities and how they REALLY work however the main part about priorities is that if you have items set for 1 which is the highest priority and you have other devices that are on the network, it's possible that the device(s) set up for priority 1 will starve out the bandwidth and internet traffic to the other devices when there all online at the same time. Thus if you lower the priority from 1 to say 10, other devices should be able to get there fair share if bandwidth. This also gives rise to the question, why not put all devices in the priority list. Yes, you can, why I added 2 PCs in the example to show that along with the high priority xboxes, PCs added in with a lower priority setting also works and this ensures that the PCs still get bandwidth while xboxes have higher priority. I presume there are various ways of setting up the priorities, just need to know what your needs are and where to place them.
FYI, after you get the router all set up, I would save off the configuration to file so that if you have to factory reset the router, all you need to do is to reload the file and you won't have to manually type in anything again. Saves time.