Maybe it would help if I told you that regexes can match two quite different things

• things such as characters e.g. \w, \s, \d etc
• things between characters, like \b, ^, $ etc

The things between characters have zero width, characters have width (generally 1, can't think of a wider example, even in Unicode)

So \b is the zero-width bit between a word character, and 'something' that is 'not-word', like a space, or a '-' or a '%' etc.