what is a subnet? explain it properly..

A subnetwork or subnet is a logical subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.
For example, 255.255.255.0 is the subnet mask for the prefix 198.51.100.0/24.
The main purpose of subnetwork is to help relieve network congestion. Congestion used to be a bigger problem than it is today because it was more common for networks to use hubs than switches. When nodes on a network are connected through a hub, the entire network acts as a single collision domain.