A prefix length describes a block, not a host
Classless Inter-Domain Routing writes a network as an address followed by a slash and a prefix length. In 192.168.10.0/24, the 24 means the first 24 bits identify the network and the remaining 8 bits identify hosts inside it. A prefix of /p always describes 2 to the power of (32 minus p) addresses, so /24 is 2 to the 8th, or 256, and /26 is 2 to the 6th, or 64. The block size is always a power of two, and every block starts on a multiple of its own size.
The tool expands any CIDR block into the pieces you need when configuring something: the network address, the first and last usable host, the broadcast address, the dotted netmask, the wildcard mask used by access lists, and the total host count.
What the numbers mean
For any prefix from /1 to /30, usable hosts equal 2 to the power of (32 minus p) minus 2. The two subtracted addresses are the network address, where all host bits are zero, and the broadcast address, where all host bits are one. Neither can be assigned to a host. The netmask is the same length as the prefix, written as ones then zeros: /24 is 255.255.255.0, /26 is 255.255.255.192, /30 is 255.255.255.252.
Two exceptions matter. A /31 has only two addresses and, under RFC 3021, both are usable on point-to-point links, so the minus-two rule does not apply. A /32 names a single host and is used for host routes and loopback entries, where the address is both the network and the only member.
Expanding 192.168.10.0/26
Expand 192.168.10.0/26. The block size is 2 to the 6th, or 64, so the range runs from 192.168.10.0 to 192.168.10.63. Usable hosts are 192.168.10.1 through 192.168.10.62, which is 62 addresses. The broadcast is 192.168.10.63, the netmask is 255.255.255.192, and 0.0.0.63 is the wildcard form used by Cisco access lists. The next block starts at 192.168.10.64.
Now the case that catches people out: a host configured as 192.168.10.75/26. Because 75 is not a multiple of 64, the address is not a network address; masking off the host bits gives 192.168.10.64/26, whose range is .64 to .127 and whose usable hosts are .65 through .126. Writing 192.168.10.75/26 in a firewall rule is accepted by most devices but means the block .64 to .127, not a single host, which is a security problem as well as a documentation one.
Where the output gets used
The common consumers are firewall allowlists, cloud security groups, route tables, VPN split-tunnel definitions and ISP address allocations. Each of those wants a different one of the outputs: a security group wants the block, an ACL sometimes wants the wildcard, a route table wants the network and prefix, and a capacity plan wants the host count.
Two limits are worth knowing. The tool cannot warn you about overlapping blocks unless you give it the other block, so check adjacent allocations by hand. And it describes IPv4 only. If you need to carve a block into smaller subnets with individual host ranges, the subnet calculator does that split; for the historic class and private-range check, see IP class finder; and for the integer form of the first or last address, IP converter.