Keywords are the most important words on your webpage. They are the words and phrases that describe exactly what you do, or sell.

Keyword research is the single most important aspect of any search optimization and promotion campaign, without exception. The keyword research process brings to light information that is both beneficial and directly applicable to many aspects of the business. Interestingly enough, keyword research is also the most commonly neglected and under-developed aspect of many search marketing campaigns.

Frequency, prominence and weight

Frequency is how often your keyword appears on the page or in its elements (Title, META Description and Keywords, visible text, H1, ALT, etc.)
Prominence is how close to the start of the tag your keyword appears. In general, a keyword that appears closer to the beginning of a page element (Title, META Description and Keywords, visible text, ALT, etc.) will make your Web page more relevant.
Weight shows the number of times a keyword appears in an area as a percentage of all words in the page: To put it simply: Weight = (Words in keyword/All words) x 100%