Bold: Bold refers to a way to make words appear in a bold font. This makes certain words, phrases or sentences more likely to be read by users who might otherwise only be scanning a page. As a result, bold text is used frequently in warnings, directions, prose, descriptions and more as a way to draw attention or emphasize certain statements.
Because of this typical usage, a search engine may place a slightly heavier weight on bold words than regular text. However, this font should be used sparingly: if you write a page of natural copy which uses the same word or phrase many times, it won't make sense or look natural to bold the word every time, especially if you are only trying to optimize your keyword usage. Similarly, emboldening several different phrases, words or sentences can overwhelming the user, especially because readers are trained to interpret bold words as yelled rather than spoken. You wouldn't overuse capital letters because you know it looks unprofessional; overusing bold font can have the same effect.
In HTML, words can be written in bold font by placing them within brackets which read "strong" and "/strong". For example, the word "water" written in this manner would appear as water.