Most characters in a pattern stand for themselves. abc matches the letters a, b and c, in that order, and there is nothing more to it than that.
Twelve characters do not: ^ $ \ . * + ? ( ) [ ] { } |. Each one means something, and each one can be turned back into an ordinary character by putting a backslash in front of it.
A backslash also builds characters that are hard to type: \n is a newline, \t a tab, \x41 the character with hexadecimal code 41, which is a capital A.
And a backslash in front of a character that means nothing special is simply that character. \q is q. That sounds harmless, and it is the reason a typo in a pattern often does nothing visible at all.