What does /24 mean?

CIDR notation is just a shorthand for counting bits. Drag the slider and watch what the slash number actually controls.

. . . /24

an address, plus a number telling you where the network stops

/0/8/16/24/32
network bits — fixed by the mask host bits — free to vary
SUBNET MASK
255.255.255.0
NETWORK BITS
24
HOST BITS
8
NETWORK ADDRESS
193.168.1.0
BROADCAST ADDRESS
193.168.1.255
USABLE HOST RANGE — for humans, not for one specific device
193.168.1.1 – 193.168.1.254 (254 addresses)

The number after the slash is a bit count, nothing more. It says: "starting from the left, this many bits of the mask are locked to 1 — the rest are 0." /24 means 24 ones then 8 zeros, which written in dotted decimal is 255.255.255.0. There's no separate concept to learn — the mask and the slash number are two spellings of the same fact.