A computer network is something through which many computers or devices get connected in a certain pattern and can communicate with each other by following some protocols. This includes all connectivity techniques like WiFi, LAN networks and any other network connectivity.
So, everything that happens online or on the internet (e.g. email, product-buying, online transaction so on) is nothing but a huge part of the network. Therefore, network security is important to protect against hacking, misuse, and unauthorized access to the files and to secure them from external threats.