Prominence is how close to the start of the area that the keyword appears. In general, a keyword that appears closer to the top of the page or area will be more relevant. However, sometimes it helps to have a keyword in the middle of an area, or even toward the end of the area.
For example, an Infoseek search for keyword "Pre-Owned Electronics" returned the following match:
Pre-Owned Electronics, Inc - Refurbished and Used Apple Macintosh Systems, The independent source for new, remanufactured and used Apple Macintosh computer systems, parts, peripherals and accessories. We offer a full line of refurbished as well as used… 98% http://www.preowned.com/ (Size 3.1K)
Note that the queried keyword, "Pre-Owned Electronics," is the first word of the site title and InfoSeek returned this site as the first match. Documents that are exactly the same, but with keywords as the second or third word in the Title will often score lower. Prominence also applies to the words within the body of the document, the headings, and other tags.
Prominence plays a critical role particularly in directory based engines such as Yahoo. Often having the keyword slightly more towards the beginning of the site description or site title will make a large difference in your ranking.
Examples of prominence:
If a keyword appears at the beginning of an area, its prominence will be 100%.
If a keyword appears in the middle of an area, its prominence will be 50%.
If the keyword appears at the beginning of the area, then another repetition appears at the end of the area, the prominence would be 50%.
If the keyword appears at the end of the area, prominence would be 0%.