Server harding is a process to make system secure and manageable.
for example.
- root login disable on ssh
- set password in single usermode
- removing unwanted kernal module
- enforcing SELinux
- remote login warning banner
- apply security patches
- Disable xinetd service
- unwanted network port blocked
- Enable audit logs
- logging is configured
Steps for hardening : https://www.cisecurity.org/cis-benchmarks/