How to read mysqld.log files? can someone tell me how to read the mysqld.log?

I'm curious about the third column. 
Because every time i start/stop the mysql it give different output. I hope someone can explain to me :).

Below is the example:

What is 18614?

2018-08-09 23:08:03 18614 [Note] InnoDB: Using CPU crc32 instructions
2018-08-09 23:08:03 18614 [Note] InnoDB: Initializing buffer pool, size = 80.0G
2018-08-09 23:08:07 18614 [Note] InnoDB: Completed initialization of buffer pool
2018-08-09 23:08:08 18614 [Note] InnoDB: Highest supported file format is Barracuda.
2018-08-09 23:10:36 18614 [Note] InnoDB: 128 rollback segment(s) are active.
2018-08-09 23:10:36 18614 [Note] InnoDB: Waiting for purge to start
2018-08-09 23:10:36 18614 [Note] InnoDB: 5.6.36 started; log sequence number 6889901435830
2018-08-09 23:10:36 7fc3935fe700 InnoDB: Loading buffer pool(s) from .//ib_buffer_pool
2018-08-09 23:10:36 18614 [Note] Server hostname (bind-address): '*'; port: 33060
2018-08-09 23:10:36 18614 [Note] IPv6 is available.
2018-08-09 23:10:36 18614 [Note]   - '::' resolves to '::';
2018-08-09 23:10:36 18614 [Note] Server socket created on IP: '::'.
2018-08-09 23:10:36 18614 [Note] Event Scheduler: Loaded 0 events
2018-08-09 23:10:36 18614 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.6.36-log'  socket: '/var/lib/mysql/mysql.sock'  port: 33060  MySQL Community Server (GPL)



What is 20239 ?

2018-08-09 23:12:23 20239 [ERROR] Plugin 'InnoDB' init function returned error.
2018-08-09 23:12:23 20239 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
2018-08-09 23:12:23 20239 [ERROR] Unknown/unsupported storage engine: InnoDB
2018-08-09 23:12:23 20239 [ERROR] Aborting



What is 100508?

2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'PERFORMANCE_SCHEMA'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'MyISAM'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'CSV'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'MEMORY'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'MRG_MYISAM'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'sha256_password'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'mysql_old_password'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'mysql_native_password'
2018-08-19 23:35:32 100508 [Note] Shutting down plugin 'binlog'
2018-08-19 23:35:32 100508 [Note] /usr/sbin/mysqld: Shutdown complete



What is 2283?

2018-08-19 23:38:15 2283 [Note] InnoDB: 5.6.36 started; log sequence number 6952861241977
2018-08-19 23:38:15 7f5dbffff700 InnoDB: Loading buffer pool(s) from .//ib_buffer_pool
2018-08-19 23:38:15 2283 [Note] Server hostname (bind-address): '*'; port: 33060
2018-08-19 23:38:15 2283 [Note] IPv6 is available.
2018-08-19 23:38:15 2283 [Note]   - '::' resolves to '::';
2018-08-19 23:38:15 2283 [Note] Server socket created on IP: '::'.
2018-08-19 23:38:16 2283 [Note] Event Scheduler: Loaded 0 events
2018-08-19 23:38:16 2283 [Note] /usr/sbin/mysqld: ready for connections.


Thank you.
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.