When you translate it out it bits it’s the number of bits that are active in the net mask

0000 0000 0000 0000 0000 0000 0000 is a /0

1111 1111 1111 1111 1111 1111 0000 0000 is a /24

1111 1111 1111 1111 1111 1111 1100 0000 is a /26

And since it’s used for matching, it follows how many there are.

Edit: typed that quickly, but to expand it further, take a /24 subnet, that is 255.255.255.0, if you look above each group of 8-bits can cover 0-255, so you in essence by having 24 bits turned on that’s what you want for an exact match. You’re basically saying “the first three octets need to exactly match”