SHOW connection_id()
It is a thread _id which you see in SHOW Prpcesslist

ID


The connection identifier. This is the same value displayed in the Id column of the SHOW PROCESSLIST statement, displayed in the PROCESSLIST_ID column of the Performance Schema threads table, and returned by the CONNECTION_ID() function within the thread.

https://dev.mysql.com/doc/refman/8.0/en/performance-schema-processlist-table.html