OS X El Capitan and later includes security technology that helps protect your Mac from malicious software.
System Integrity Protection is a security technology in OS X El Capitan and later that’s designed to help prevent potentially malicious software from modifying protected files and folders on your Mac. System Integrity Protection restricts the root user account and limits the actions that the root user can perform on protected parts of the Mac operating system.
Before System Integrity Protection, the root user had no permission restrictions, so it could access any system folder or app on your Mac. Software obtained root-level access when you entered your administrator name and password to install the software. That allowed the software to modify or overwrite any system file or app.