Complex sentence: In computing, hardening is usually the process of securing a system by reducing its surface of vulnerability, which is larger when a system performs more functions; in principle a single-function system is more secure than a multipurpose one.
Split into simpler sentences: In computing, hardening is usually the process of securing a system by reducing its surface of vulnerability. A system has a larger vulnerability surface the more functions it fulfills; in principle a single-function system is more secure than a multipurpose one.