What does #abc mean in a css code

The [abc] expression is used to find any character between the brackets.

The characters inside the brackets can be any characters or span of characters:

[abcde..] - Any character between the brackets
[A-Z] - Any character from uppercase A to uppercase Z
[a-z] - Any character from lowercase a to lowercase z
[A-z ]- Any character from uppercase A to lowercase z
Tip: Use the [^abc] expression to find any character NOT between the brackets.

The blank would be 'natural'.