If you go to the back pages of a reference book, you'll find a list of key words and which page of the book those key words are on. This is called an index, and while tremendously more sophisticated, Google's index is a digital version of this concept.
The crawlers (see Part 3) look at the words on a webpage and send them to the Google index, along with other data, such as where the word is on the page, whether the word is part of a title, or in the description of an image (called "alternative text" or an "alt tag").
The index of words is obviously important in matching a web page to the word or phrase that is being searched, but it is only one of many factors considered in the ranking. For example, if you type in "plumbers" in the search box, Google goes to the index to find all of the web pages that have the word "plumbers." So, if you have that word on your web page, Google knows it's a match. But how good a match?
Google understands that you are not actually looking for the word plumbers, but rather, are looking for a plumber, in a specific location, that you can call to repair a broken water pipe or install a new bathroom sink. So in that indexing process, Google is also looking for words or phrases related to "plumbers," like "broken water pipe" or "new bathroom sink." They are also looking for a web page that has a location, a phone number, and maybe a paragraph about what to do if a water pipe breaks.
The bottom line is that while words are important, it's never one specific word no matter how many times you repeat it that is going to get you on the first page of search results.