When your site for some reason returns codes 5** or explicitly displays an error message on the page, the error log, aka error_log file, can help you diagnose the problem. It stores the history of PHP errors.
There is no particular format because errors are generated by a particular script: therefore, the errors may be super detailed or just dry text without much detail.
But in any case, the text of the error will be able to shed some truth and help find the reasons for this behavior: either with the help of documentation or through Google.
Usually, the text of the error itself will indicate with which file and line of code in it the error occurred.
You can find the error log in the hosting control panel, "File manager" ➡️ "public_html" (for the main domain), or in the folder with the domain name for additional domains or subdomains.
There you should find a file named error_log.