The <b> tag defines text that will be displayed in bold format. The text marked as bold does not convey any special importance. There are few other tags like <strong>, <emph> and <mark> that serves similar purpose and it is preferred to use <b> tag as a last option.
Common mistakes while <b> tag includes marking it as header but instead the heading tags - h1-h6 - should be used instead. Also, <strong>, <emph> and <mark> tags have semantic meanings which the <b> tag does not have. Therefore, it is better to limit the use of <b> tag and instead use the style font-weight:bold instead.