You can use the priorities to organize the job queue - turns in which your submitted jobs will run. You can increase or decrease the priority number for each job that is awaiting its turn to be rendered ("Starting" status in Envoy). You can't shuffle jobs with other statuses like "Running". See the lists of known limitations and overall rules below and the working example at the bottom of it.

KNOWN LIMITATIONS AND OVERALL RULES:
SORTING:
- the higher the number the higher the position in the queue
- jobs with the same priority will run according to their submission time
- all running and starting (queued) jobs will be sorted according to their priorities
- all finished or stopped jobs will be queued at the bottom according to their submission date
KILLING AND RESTARTING JOBS/FRAMES:
- when you are maxing out node limit with the high priority job and stop/kill some frames that won't release the pool for other jobs - that allows you to restart those frames within the current pool
- you can't put queued job before the one that is already running - the only way to do that is to kill the running job
CHANGING ENVOY'S GLOBAL SETTINGS:
- changing the Plan from Economy to Rush for example will grant you additional nodes but it might disrupt the way you have set your jobs with priority numbers so the queue may not exactly evaluate as expected

EXAMPLE:
We have 3 jobs - 1 on the top of the queue is running on 5 nodes and since we are on Economy plan we only get 5 nodes at our disposal so other jobs are waiting for it to finish.
Use the UP/DOWN arrows you see on the far left to each job in order to adjust the priority of the queued jobs. Higher number means higher position in the render queue:
After adjusting the priority the view will refresh - it will do that each time the given priority value is adjusted. As the actively running job finishes the one with highest priority starts next and the finished ones are placed at the bottom according to their submission date:

Please remember only the queued jobs can be sorted so as soon as the job changes state from "Starting" to any other the priority disappears. Now the Priority 2 job is running:

As Priority 2 job finishes Priority 1 starts: