Para citar wikipedia :
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 that it does; in principle a single-function system is more secure than a multipurpose one. Reducing available vectors of attack typically includes the removal of unnecessary software, unnecessary usernames or logins and the disabling or removal of unnecessary services.
Normalmente existem guias pelo fornecedor do sistema operacional. Procure por exemplo no guia Debian: link