Keyword prominence refers to how prominent keywords are within a web page. The general recommendation is to place important keywords at, or near, the start of a web page, sentence, TITLE or META tag.
Keyword proximity refers to the closeness between two or more keywords. In general, the closer the keywords are, the better.
For example:
How Keyword Density Affects Search Engine Rankings
How Keyword Density Affects Rankings In Search Engine
Using the example above, if someone searched for "search engine rankings," a web page containing the first sentence is more likely to rank higher than the second.