It computes network address, broadcast address, first/last usable IP, and the number of hosts for an IP and subnet mask or prefix.
CIDR notation writes an IP with a suffix like /24 indicating the number of network bits (e.g., 192.168.1.0/24).
A /24 network has 256 IP addresses, of which 254 are usable hosts (excluding network and broadcast addresses).
Some subnet calculators support IPv6; check your tool version — IPv6 uses much larger address space and different prefix sizes.