An organic medium is when a user looks up your website using a search engine (Google, Bing, Yahoo, etc.) and is also considered unpaid search.
Search Engine Optimization (SEO) may be required for your website to be ranked higher on the SERPs (Search Engine Results Page). If your organic results are higher than the other source/mediums, that means that users are looking for your car dealership by typing the name into the search engine.
Paid advertisements help you get your name out there, and even though users may not always click on your advertisements, your name may already be in their minds. They may remember your car dealership’s name the next time they are looking for a vehicle. This eventually leads to organic searches.