Mod_security is an apache module that helps to protect your website from various attacks.
It is used to block commonly known exploits by use of regular expressions and rule sets.
Nowadays many hosting providers use their own mod security rules on the server, so users don't need to apply their own security mod.
Mis configuration & using of Mod Security in your hosting may cause the error 403, 404 or 500, which is nothing but mis matching of your mod scurity rules with server rules.
Mod_security can be easily disabled with the help of .htaccess. Here's the code that you will need to enter in .htaccess file to disable mod security.
<IfModulemod_security.c> SecFilterEngine Off SecFilterScanPOST Off </IfModule>