Anybody who uses search engines to find out information uses phrases to find the information they are seeking. These phrases to them, best describe the information they seek. So,  if the are looking for information on what to look for when buying a second hand car  for example you might use the term "buying second hand cars", or "buying used cars", or "how to buy used cars", etc.  Each of these terms is commonly known as a ‘keyword’, and searching for the most used keywords, or even lesser used keywords, is big business on the internet.

If you have a website you can optimize it only for one of these keywords if you want the bets results. Many people make the mistake of trying to optimize a webpage for all the keywords they can think of, totally oblivious to the fact that if they have 20 keywords then they should have 20 web pages, each optimized for one keyword.