How to read mysqld.log files?
It's a thread_id as per https://dev.mysql.com/doc/refman/8.0/en/error-log-format.html :

This log writer produces the traditional error log output. It writes messages using this format:


  timestamp thread_id [severity] [err_code] [subsystem] message


The [ and ] square bracket characters are literal characters in the message format. They do not indicate that fields are optional.