I would like to know when i use show spanning-tree | begin interface commande. what does the column: Prio.Nbr mean
This is the port priority. It is composed of the priority (which by default is 128) and the number of the port. So if you have port Fa0/5 with its default priority, it would have a Prio.Nbr of 128.5.
This number is used to determine which port on a switch will be blocking and which will be forwarding ONLY for situations where there are multiple links between the same two switches.
The port priority and internal port combination come into play as tiebreakers when a switch is choosing it’s root port as follows: the first choice is the local port that’s part of the least root cost path. If those costs tie, then the choice is made based on:
lowest neighbor bridge ID, if tie, then… (if this is a tie, then the two switches are connected using multiple links)
lowest neighbor port priority, if tie, then…
lowest neighbor internal port number
Note that items 2 & 3 only come into play when two switches connect to each other with multiple links, because in that case, a switch is receiving hellos on more than one port from the same neighboring switch so the bridge ID’s tie.
I hope this has been helpful!