The b element is used to markup special text parts, without conveying any extra importance to it. This could be keywords on a page, and product or company names in a review.
How to use b
<p>This is just a very long paragraph of text, where i mention <b>Brugbart</b> as a cool website.</p>
The below would be an example of incorrect use, instead you can use strong in such a case.
<p><b>404 ERROR!</b> The page was not found!</p>