It originated in the English Civil War, when Royalist prisoners of war were sent to the city of Coventry. The city's inhabitants were known to be staunch Parliamentarians and treated the prisoners coldly, refusing to speak to them or interact with them in any way.
So, to be "sent to Coventry" is to be treated with the same cold shoulder and complete isolation that the Royalist prisoners received.
Here are some examples of how the idiom is used:
* "After the argument, they sent each other to Coventry."
* "The team sent him to Coventry after he broke the rules."
* "My boss sent me to Coventry after I made a mistake on a big project."