The ErrorLog directive in a web servers configuration file (e.g. httpd.conf) is used to define the location of the error log, something like this.

ErrorLog logs/error_log

With this configuration, the error log would be located at:

<web_server_root>/logs/error_log

As the name implies, the error_log will capture events classified as an error. Here is an example of an error in error_log.

[Fri Mar 20 21:21:28.084472 2020] [unique_id:alert] [pid 3538:tid 139812487051072] (EAI 2)Name or service not known: AH01564: unable to find IPv4 address of "server1"
AH00016: Configuration Failed

When using the apachectl command to start or restart the web server, the -e option followed by a log level (such as debug) can be used to append additional details to the error log.