Bold text is used for emphasis to make certain words and phrases stand out from surrounding text. HTML uses tags like <b> and <strong> to make text look bold.

Table Header <TH> tags are also displayed as bold text in most browsers. 

<b> and <strong> may look the same but there's a slight difference. <b>  tell the browser what the text should look like while <strong> indicates stronger emphasis, which can be conveyed by screen readers for people with accessibility issues. 

According to Matt Cutts, <strong> and <b> are exactly the same when it comes to ranking and optimizing pages for Google.