Compare whole IPv4 octets

IPv4 has 32 bits in four octets. An octet is eight bits written as one
dotted decimal number from 0 through 255. A /24 fixes 24 leading bits,
exactly the first three octets. Compare complete numbers: 5 is not 50.

A subnet mask marks fixed bits with 1 and variable bits with 0.
For /24 this is 255.255.255.0. Eight bits remain free. Each bit has two
possible values, giving 2 to the power 8 = 256 total addresses. That
total includes network and broadcast positions; it is not an ordinary
usable-host count. Other prefixes need their own membership rules.

Prefix cards. Original fictional addresses. Not your computer's addresses.

Worked prefix 192.168.40.0/24
The prefix length is 24 bits. In this octet-aligned example the first three octets stay 192.168.40.
192.168.40.10 is inside.
192.168.41.10 is outside because the third octet differs.
The mask with those same 24 one-bits is 255.255.255.0.
Eight bits remain, so the prefix covers 256 addresses. This counts the whole block rather than only assignable ordinary-host addresses.

Activity prefix 10.8.0.0/24
10.8.0.15   compare the first three octets
10.8.1.15   compare the first three octets
10.9.0.15   compare the first three octets
Predict before reading the check. Check: 10.8.0.15 is inside. The other two are outside.

A /31 or a /32 does not use this lesson's three-octet habit. Stay with the stated /24.