In priority view, you can organize your projects and resource allocations by priority. 

Work at the top of the plan has a higher priority than work at the bottom of the plan.

The priority of work is used to decide when to show over allocation alerts. Here is a deeper dive into the process of deciding which work is over allocated.

To change the priority of work in the plan

If you want to change the priority of work in the plan click and hold the work that you want to move and drag it to a new priority location in the plan. 

Select a resource assignment row to just move a single piece of work or select a project row to move all the work in that project group.