What Is ModSecurity?
ModSecurity is a web application firewall that can work either embedded or as a reverse proxy. It provides protection from a range of attacks against web applications and allows for HTTP traffic monitoring, logging and real-time analysis. 
mod_security is an Apache module (for Apache 1 and 2) that provides intrusion detection and prevention for web applications. It aims at shielding web applications from known and unknown attacks, such as SQL injection attacks, cross-site scripting, path traversal attacks, etc. 
mod_security is an Apache module designed as a sort of web application firewall. It’s most useful for preventing SQL Injection and Cross Site Scripting (or XSS). 
It is also an open source project that aims to make the web application firewall technology available to everyone. 
SQL injection is a technique that exploits a security vulnerability occurring in the database layer of an application. The vulnerability is present when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and thereby unexpectedly executed. It is in fact an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another.
Cross-site scripting (XSS)
Cross-site scripting (XSS) is a type of computer security vulnerability typically found in web applications which allow code injection by malicious web users into the web pages viewed by other users. Examples of such code include HTML code and client-side scripts. An exploited cross-site scripting vulnerability can be used by attackers to bypass access controls such as the same origin policy. Vulnerabilities of this kind have been exploited to craft powerful phishing attacks and browser exploits. As of 2007, cross-site scripting carried out on websites were roughly 80% of all documented security vulnerabilities. Often during an attack "everything looks fine" to the end-user who may be exposed to unauthorized access, theft of sensitive data and financial loss.
5. Metalink Note ID: 456388.1 - How to enable and test mod_security in HTTP Server