Command : spanning-tree vlan <vid-list> priority <0...15>
Now there are two things here,
One is the priority and the other priority value.
Root bridge is the one with lowest BRIDGE ID.
Bridge id = priority (2 bytes) & Mac address (6 bytes).
Default priority is set to 4096.
The 0-15 is the priority value that is a multiplier to this default priority.
So for example if you set this as 1, your priority is 1*4096.
Now leaving it at this will make sure that for this vlan id(one vlan or multiple vlan) this switch will be selected as root bridge provided you change the priority of other switches for this vlan to a higher value (2* or 3*).
This is a tech forum and not the official Aruba support, if you are looking for the official Aruba support, you can reach them at support.arubanetworks.com.