Reading it
In 192.168.1.0/24, the first 24 bits are the network and the remaining 8 identify hosts. That gives 256 addresses, of which 254 are usable. A /22 holds 1,024 addresses, and a /30 holds 4, with 2 usable.
Why it matters
CIDR lets networks be sized exactly instead of in fixed classes, which conserves addresses and simplifies routing. It is used in firewall rules, cloud networks and routing tables.
Try it
The Subnet Calculator returns the mask, range and host count for any prefix, and the guide shows worked examples.
Common misunderstanding
A bigger number after the slash means a smaller network. A /30 is much smaller than a /16. See IPv4 for the addresses themselves.