A keyword as any word that someone will use when searching the internet.

Search engines like Google, YaHoo and Bing are like a giant index in the back of a book. When you search for something on the web, the search engine uses the words that you type to search through an index it has created. Search engines maintain an index of web pages on the Internet and display the results of your search ordered with what it believes to be the most relevant web pages on top.

Tyically when keywords are mentioned in terms of search engines it is because the owner of a website wants more visitors to their site. More visitors should arrive at a site if they are finding the page on a search engine.