What happens if someone becomes Excommunicado?

Excommunicado basically means that Wick becomes persona non grata and loses all access and privileges to underworld resources.

What I did not understand is, why John is running for his life?

Clearly, John doesn't have any bounty on him, why would anyone risk their lives to kill a person like THE JOHN WICK?

He does have a bounty on him, a HUGE one.

There's a $14 million hit ordered on him by the Chinese & Russian seats on the High Table.

As for the reasoning, it was a concious decision by the films creators

Keanu Reeves commented....

^{CinemaBlend via ScreenRant}

“We wanted to come to a place that John literally had to fight for his life, and that he’s starting to become a little anti-establishment. And that’s really where the character starts to develop. It’s like the rules and world that he lived by, aren’t starting to work for him, and they’re not working! There are people who are out to get him because of [his] transgressions. And we liked that idea because we like John – when I say ‘we’ I mean the producers, the writer, the director — we like to see him suffer!”

“We wanted to put him in another vulnerable… ‘How’s he going to get out?! How’s he going to survive?! What’s it mean?! What’s he going to do?!’ I don’t know! We have some ideas. Maybe he needs to go to Jerusalem! Or he’s in the middle of the desert. Or whatever!”

This is essentially excommunication or exile. From wiki:

Excommunication is an institutional act of religious censure used to deprive, suspend, or limit membership in a religious community or to restrict certain rights within it, in particular receiving of the sacraments. The term is often historically used to refer specifically to Catholic excommunications from the Catholic Church, but it is also used more generally to refer to similar types of institutional religious exclusionary practices and shunning among other religious groups.

This isn't limited to religions of course. Brotherhoods of many types use this. When excommunicated, you are no longer part of the group, you can't be supported by it, and no one is supposed to help you. Often trying to interact will result in harsher penalties.

Before excommunication, Wick had a bounty that anyone who was greedy or foolish enough could try to attack him if they want to. Excommunication isn't voluntary though. No one else is allowed to talk to Wick without risking punishment themselves.

Frankly, an organization of assassins, who risk being outed, would equate excommunication as a death sentence anyway.