Google is in the business of advertising. When people open Google, they are looking for something with intent. They want to find something to buy, something more about that something, something about its pricing, or something about its reviews. But, they plan to buy something. So, when they type words into the search panel, they are spelling out what they want.
If you want to place ads on Google or its network, you need keywords that are in demand in terms of popularity and density of use. Google AdWords mines those words and phrases and auctions them off. The system lets you budget the advertising and pay only when people click on the ads. The AdWords produce relevant ads with keywords that people use when searching. Visitors trigger a Google ad when you browse for that keyword. Your ad, for instance, would appear on a right margin panel on the monitor screen when the visitor browses for a related AdWord.
Google continually runs algorithms creating a Quality Score for keywords and phrases. This score ranks how well they relate to what people are looking for and how likely they are to click through. That Quality Score creates a best bid position in an auction for those desirable words. Calculating and staying on top of these complicated auctions takes some sophistication beyond the know-how of most business owners.