Network Security is an over-arching term that describes that the policies and procedures implemented by a network administrator to avoid and keep track of unauthorized access, exploitation, modification, or denial of the network and network resources.
This means that a well-implemented network security blocks viruses, Malicious Software (Malware), hackers, etc. from accessing or altering secure information.
Source: https://www.techopedia.com/definition/24783/network-security