The network address: applying the mask
For 192.168.1.10/24, the /24 prefix means the first 24 bits are the network portion — that gives a network address of 192.168.1.0, a broadcast address of 192.168.1.255, and 254 usable host addresses (192.168.1.1 through 192.168.1.254).
A smaller subnet, fewer usable hosts
The same IP address with a /28 prefix instead — a smaller subnet — gives a network address of 192.168.1.0, a broadcast of 192.168.1.15, and only 14 usable hosts (192.168.1.1 through 192.168.1.14). A longer prefix reserves more bits for the network portion, leaving fewer bits (and so fewer addresses) for hosts.
The /31 exception: point-to-point links
Normally, both the network and broadcast addresses in a subnet are reserved and unusable. But a /31 subnet (10.0.0.0/31) is a special case under RFC 3021: both addresses in that 2-address block — 10.0.0.0 and 10.0.0.1 — are usable, with no separate broadcast address at all, since a link connecting exactly two devices has no need for one.
Why network and broadcast addresses are normally reserved
The network address identifies the subnet itself, and the broadcast address is used to reach every host on the subnet at once — neither can be assigned to an individual device, which is why a /24 subnet's 256 total addresses yield only 254 usable ones.