prefix length
The /24 after an address: how many leading bits name the network.
The prefix length (CIDR notation, as in 192.168.1.0/24) is the number of leading address bits that identify the network; the remaining bits number the hosts.
Examples
In 192.168.1.37/24, the /24 says the first 24 bits (192.168.1) name the network and the last 8 name the host.
A /26 leaves 6 host bits, so the subnet holds 64 addresses.
See also