Bolding should only be used to emphasize key points. Use bolding for keywords that are used for Google ranking. Unless relevant, you don't want to bold LSI keywords (synonyms), as that would be overkill.
Use bolding for definitions or key points in a paragraph. Only a few words should be bolded per paragraph. An entire paragraph should not be bolded (unless it is used as a Snippet paragraph, as mentioned in a previous blog post), as this will signal to Google bots that there is a coding error.
Bolding cues the reader's eyes to the right words, the main points, in a paragraph, so use it to help guide the reader along. The majority of readers will not read every word in a blog post, so make sure you guide them through the text with cues like italics and bolding.
Examples of bolding and more info on how to bold can be found in the writer guidelines! Please ASK if you have any questions.