What does excommunication mean in John Wick

What happens when someone becomes an excommunicado?

This is essentially excommunication or exile. From the wiki:

Excommunication is an institutional act of religious censure that serves to deprive, suspend or restrict membership in a religious community or to restrict certain rights within that community, in particular the reception of the sacraments. The term is often used historically to refer specifically to Catholic excommunications from the Catholic Church, but it is also used more broadly to refer to similar types of institutional religious exclusion and avoidance among other religious groups.

Of course, this is not limited to religions. Brotherhoods of many kinds take advantage of this. If you are excommunicated, you are no longer part of the group, cannot be supported by it, and no one should help you. Often times, trying to interact leads to harsher penalties.

Before the excommunication, Wick had a bounty that anyone greedy or foolish enough was to attempt could to attack him, if he wanted to. However, excommunication is not voluntary. Nobody else is allowed to speak to Wick without risking punishment themselves.

In all fairness, an organization of bombers at risk of being withdrawn would equate excommunication as a death sentence anyway.