More information about the tag body (+bold)
Search engines such as Google primarily analyse the continuous text of a web page in order to determine how relevant it is for a keyword that is used in a search. It goes without saying that the most important keywords in the text must occur but not excessively: this is regarded by search engines as spam and is punished with a lower value and a lower position in the search results. Repeating the same keywords too often is also irritating for human visitors: they regard the information as not relevant as a result and are more likely to click off the page.
A good mix of keywords and words related to it is ideal. A useful exercise in preparing to write good web content is as follows: use Google to search for high scoring web pages for certain keywords, analyse the amount of text they contain and which keywords and related words are used. But do not be tempted to copy: intelligent search engines pick up on that (Google for example) and will ascribe a lower value. A mathematical approach to writing web page content is also not the best idea: it can come across as forced for human visitors and they will tend to click off. Human visitors like good information: let your valuable content show that you are a specialist and thus win the confidence of your visitors. This increases the chance that they will actually do what you are expecting and fulfil the very purpose of the website: download a file, complete a contact form, call you or, ideally, order products or services and become a new customers.
The fact that human visitors often scan the text on a web page - they read the title and the first paragraph, quickly view the images, read the sub-headings and then scroll before deciding whether or not to actually read it, means it is useful to 'highlight' important words in the text, e.g. by making them bold. This ensures that these words attract the attention of visitors that quickly run through the text. Search engines such as Google take limited account of text elements with a special layout (bold, large, underlined, etc.) and that is why it is useful to give the most important keywords a special layout. But beware: do this with caution in order to ensure that search engines do not regard it as spam.