Hello Internetbrother,
Modsecurity is what they call an application firewall. It has rulesets and scans incoming traffic. When that traffic meets one of those rules it blocks it.
It's a good thing to have as it will even block some known applications hacks and unused protocols but you have to be very diligent for a while checking the log to make sure that legitimate traffic is not being blocked and then you have to add exceptions to the rules if it is.
Hope that helps!