Mod_security is a web application firewall (WAF) that protects on web application layer that is being the most target for a website.
It provides protection over the webserver by looks for known malicious request as well as monitor outbound data and block information such as leakage of detailed error messages,social security number or even credit card numbers.
Effects on website:
If mod_security match the rule, which means it hit, the access to the website will be restricted or an Internal server error might occur.