If you put a keyword in google search bar, or search anything with your voice, you will notice the top 4 results have a small notice with them on which it is return that they are ads, the top 4 results are paid advertisements, and when people click on those paid results, then the google charge the companies who have placed those advertisements there. While if you somehow with Search Engine Optimization bring your website on front page of Google for any search term, and start getting traffic naturally from Google without paying anything, then it is known as organic traffic, it is free of cost.