CIDR (Classless Inter-Domain Routing) is a method used to define the IP address range for a network. It specifies how many IP addresses are available in a network or subnet.
For example, 10.0.0.0/16 is a CIDR block where 10.0.0.0 is the network address and /16 represents the subnet mask. In AWS, CIDR blocks are used when creating VPCs and subnets.
In simple words, CIDR defines the size and range of IP addresses that can be used in a network.